Well... nobody's offered any commentary here, so I figured I might as well post the remaining classes/subclasses. I don't know if that's what you were waiting for or if this forum is just dying.
Druid
Druids are an uncommon class in the Lands of Mist, where most people fear the wilderness that lies beyond their doors and huddle in the safety of the cities where they can. Only in domains with strongly rural populations tend to produce druids, and even then, they are often held at bay. Only in the wildest domains are druids truly welcomed and respected, for their skills with the natural world can mean the difference between life and death.
Circle of the Land
Druids following the Circle of the Land are the most common variety of druid, and can be found in all environments. Druids of the Desert roam the burning sands of the Amber Wastes, whilst druids of Forest and Mountain work together in Barovia or Tepest. Rare indeed is the terrain that doesn't have some druidic representation somewhere. Even the Underdark is possible, if your DM agrees; such a druid may be one of the calibans of sunken Timor, or may have struggled to understand the sunken world of the sewers in Richemulot.
Circle of the Moon
These druids, in contrast to their brothers, are a minority in the lands of mist. Even druids are wary of embracing the power to shift form too readily, fearing it may invite whatever strange feral intelligence it is that they can feel inhabiting the land around them into their bodies. Some even believe that this is the ultimate source of therianthropes, especially the savage werewolves of Verbrek. Druids of the Moon are generally most likely to arise in domains that have no major therianthropic presence, minimizing their inherent risk of being mistaken for creatures of the night.
Fighter
In all lands, warriors are ubiquitous, and the Lands of Mist are no exception to this. Wherever one goes, so long as there is some form of civilization, there will be fighters there.
Banneret
Bannerets are an uncommon class of fighter, at least in their conventional form. These warriors excel in the use of their skills to inspire and bolster their allies, and though such traditions are more associated with noble knights, as in Darkon, they find uses all over. Falkovnia is also a likely source of bannerets, even if the classical version of the domain is likely to foster a particularly cruel and intimidating breed. A samurai from Rokushimo Taiyoo is a possible banneret, as is a bandit leader or a mercenary captain Nova Vaasa or Invidia.
Battle Master
With their approach to fighting being based on the understanding and mastery of combat tactics, battle masters tend to be most common in domains that either actively uphold military campaigns and forces, or which have well-documented military histories. Thusly, while many battle masters in the Core hail from the militaries of Falkovnia, Darkon, Borca, Lamordia and Richemulot, they may also potentially hail from domains like Forlorn or Barovia.
Champion
As the most simple and straightforward breed of advanced fighter, there literally is no end to the possibilities where such a warrior could come from. The captain of a volunteer militia in Barovia, a brutal street gangboss from Nova Vaasa, a former gladiator, anything is possible.
Cavalier
These fighters only emerge in domains where mounted combat is upheld as viable and practiced in the modern era. Darkon, Falkovnia, Invidia, and Nova Vaasa serve as the main sources for Core-native cavaliers.
Eldritch Knight
These are unquestionably the rarest of all breeds of fighter in the Realm of Dread. With the fear the common folk have of arcane magic, fighters who are willing to embrace spellcraft to heighten their abilities are extremely rare, and rare indeed are the domains that produce them in serious numbers. Darkon and Hazlan are the most likely candidates for Core domains that regularly produce eldritch knights, given the prominence both domains place on arcane spellcraft. Talk it over with your DM before trying to create an eldritch knight native to Falkovnia; canonically, Drakov's prejudice against magic would prohibit such a character, but the DM may allow you to play a candidate from the "super soldier" training programs, or they may be using an alternative version of Falkovnia, such as Die Kaiserreich from Quothe the Raven #22.
Scout
Scouts serve as a tougher, more durable analogue to the ranger, and so can be found in similar domains; wherever the wilderness makes learning to fight with it rather than against it more effective, there will be scouts. Indeed, scouts are likely to take the place of rangers in some countries where the rangers' traditionally magic skills are disregarded or unwanted, in addition to or instead of the "magicless" branch of ranger. Lamordia is the perfect example of a domain from which scouts may appear.
Monk
In most of the Lands of Mist, monks in their traditional form are a rarity; only the domains of Hazlan, Rokushima Táiyoo and Sri Raji regularly produce adventurers of this stripe. However, alternative flavorings of monk can potentially emerge in other domains; the Celebrants of Paridon, for example, produce many trained fighters who learn to focus body and mind together to achieve miraculous feats.
Way of the Four Elements
One of the rarest of the monk styles to be found in the Lands of Mist, monks practicing the Way of Four Elements will most likely hail from Hazlan, whose native martial artists assimilate strange arcane elements into their fighting styles and traditions. Monks who practice only one elemental style may be found as the elite warriors of small cults and philosophical groups. The ninjas of Rokushima Táiyoo may include such exotic and openly magical practices amongst their sects, but they would be a distinctive rarity even in that group.
Way of the Long Death
Brutally effective as assassins, monks of the Long Death are most likely to come from Sri Raji, members of the dread cult of Kali that holds sway in that domain. Rokushima Táiyoo may also house monks of this style as part of the dread clans of ninja assassins, where they likely compete for prestige and power amongst themselves with their "brothers" of the Way of the Shadow.
Way of the Open Palm
The iconic form of monk, these monks are the most commonly seen in the lands of the Core. These monks can be found in any domain that supports monastic or pseudo-monastic training, including Hazlan, Sri Raji, Rokushima Táiyoo and Paridon.
Way of the Shadow
The largest source of these dread assassin-spies is unquestionably the domain of Rokushima Táiyoo, where they form the backbone of the infamous ninja clans of that domain.
Way of the Sun Soul
This is undoubtedly the rarest form of monk to exist in the Realm of Dread; as powerful as they are against the undead, their philosophies are mostly incomprehensible to those native to the Lands of Mist. Perhaps the largest concentration of Sun Soul monks is to be found, ironically, in Barovia, where they form the carefully hidden and trained elite warriors of the secret society of Dawnslayers, a militant anti-undead wing of the Morninglord faith. With its knack for assimilating outlanders and prevalent fear of the undead, Darkon may likewise possess small orders who practice the Way of the Sun Soul. DMs may agree that this philosophy could be an exotic branch of the practices and skills developed by the Celebrants of Paridon as well.
Paladin
Warriors who swear loyalty to a guiding power or principle with such fervor that it imbues them with mystical powers, paladins are a rarity in the Realm of Dread; not unheard of, but so rare that many individuals believe they are myths and legends. But they are very real, and can come from almost anywhere.
Discuss with your DM before taking the Paladin class; this class has changed the most since the days of 2nd and 3rd edition, but even then, many DMs refused to allow the class due to it being unfitting for a setting like Ravenloft.
Oathbreaker
Sworn to the service of the foulest powers, oathbreakers are both extremely rare and alarmingly common. They are rare in that it is extremely unlikely for a paladin to attain its powers in service to darkness from the beginning - the iconic blackguard may sometimes appear in the service of a particularly powerful Darklord such as Azalin of Darkon or Strahd of Barovia, or as the champion for a mad cult of fiend-worshippers. They are common in that in the Lands of Mist, many paladins find themselves ultimately falling from their goals and being embraced by the darkness, no matter their origins.
Discuss with your DM before trying to play this class; while some DMs will allow for Bryonic Heroes or characters accepting their inevitably tragic outcome in exchange for the power to fight evil on its own terms, others will reject playable Blackguards as anathema to their vision of Ravenloft heroes.
Oath of the Ancients
Undoubtedly this is the rarest of all paladins, for it requires swearing bonds to the fae powers of nature that the vast majority of mortals fear and shun in the Realm of Dread. Only two domains produce these paladins with any regularity. Those reared by the Arak and Sith of the Shadow Rift often become "fey-knights", swearing oaths of allegiance to their adoptive patron and receiving mystical powers in kind, adventuring on their behest or to prove themselves worthy of their ultimate acceptance into the society of the fae. In Forlorn, where the culture is dominated by druids, a small number of warriors swear themselves to the protection of nature against the evil forces defiling it, an oath that bestows them the power of this pledge.
Oath of the Crown
More common than most other oaths, these paladins emerge from domains with strong autocratic traditions, those realms with traditions of mighty leadership and aggressive expansionism that can reward a reverence for one's rulers. Paladins of the Crown march under the banner of the Iron Faith, fighting for domains such as Nova Vaasa and Hazlan. Others come from Darkon, sworn to the service of the mighty Azalin Rex. In far-distant Nidalia, paladins of this sort emotionlessly serve the cause of the mad knight-queen Elena Faithhold. Even Falkovnia may potentially hold paladins of this stripe in the ranks of its ever-restless army.
Oath of Devotion
The classic form of the paladin, the kind that all people think of when the term is mentioned, and assuredly so rare as to be almost mythical. The faiths of the Morninglord and, to a lesser extent, Ezra occasionally produce such mighty holy champions.
Oath of Vengeance
More than any other breed, these paladins proliferate in the Realm of Dread. Harried and harrassed by cruelties both supernatural and mundane, many call out into the night for the strength to avenge themselves or their people. And something in the darkness responds, eagerly sharing its strength and watching as they turn it against that which catches their attention.
Ranger
In a land with the wilderness as untamed and widely spread as the Realms of Mist, it should be no surprise that rangers proliferate, for all that their more mystical powers can often alienate them from those too afraid to fight the darkness. Though rangers may originate from almost any domain, they are particularly common in the more rural ones, such as Barovia, Kartakass, Verbrek, Vorostokov, or Nova Vaasa.
Beastmaster
Beastmasters are only slightly less common than their hunter brethren, being more likely to occur in regions where animals suitable for taking as companions exist. Though the iconic beastmaster uses their animal companion to aid them on the battlefield, other uses do exist; Nova Vaasan and Har'Akiri beastmasters in particular have a tendency to bond with strong steeds to carry them into battle. A beastmaster's choice of animal companions is often shaped by the domain they hail from; a Barovian or Kartakan beastmaster is unlikely to take a wolf as a companion, given the fearful reputation such animals have in their homeland, whilst a Falkovnian beastmaster will usually take a loyal raptor.
Deep Stalker
With the general lack of deep cavernous environments that shapes these rangers, Deep Stalkers are a rarity amongst their brethren - but not impossible. In Richemulot, elite bands of deep stalkers may roam the sewers, striving desperately to hold back the seething tides of wererats and their verminous allies. In Zherisa, meanwhile, Paridonian deep stalkers try to stem the tide of horrors rising from the caverns below the streets and Timorian deep stalkers work desperately to feed and protect their tribes.
Hunter
Wherever the ranger exists, so too does the hunter exist, the most iconic of their kind.
Magicless
The magicless alternate class for rangers is a breed of ranger likely to exist in lands that have a serious need their skills, but which likewise particularly shun the wilderness skills. Lamordia is the most logical land for such a ranger to call home. Discuss with your DM before taking this class; they may prefer to replace the Magicless Ranger with the Fighter's Scout archetype instead.
Rogue
If any class in the Lands of Mist can be considered as ubiquitous as the Fighter, it is the Rogue. Where people come together, rogues are born, seeking to take what they want from the world by wits and cunning.
Arcane Trickster
Although one might expect arcane tricksters to be a rarity in the Realm of Dread, given the general aversion most have for arcane magic, that is not the case. Rogues already exist on the edges of civilisation, and as superstitious as they are, many are already considered damned, and thus willing to risk the use of magic to empower themselves. Arcane tricksters generally proliferate in realms that have strong enough traditions of written magical lore that they can learn such tricks, such as in Darkon or Dementliue - they are particularly common in the latter, where illusionists and enchanters proliferate as "mere" entertainers - but so long as they have at least a mentor figure, they can conceivably come from anywhere.
Assassin
When killing is profitable, there will be those who rise to fill the niche. Assassins can arise from all over, and whilst the stereotypical assassin hails from a more "civilized" domain, where those who wish others dead will call upon others to make it happen, this archetype has greater versatility than that. Any rogue who has learned to master the arts of killing, whether as an elegantly lethal knife-for-hire or a back-alleys mugger, falls into this subclass.
Mastermind
These rogues usually hail from more urbanized domains, for their natural niche is as the leader of criminal groups and as the center of webs of plots and plans. More rural "Masterminds" are typically gangbosses or bandit chieftains who rely on wits rather than brutal enforcement to lead their bands to successes.
Swashbuckler
With flashing blade and cunning wit, the swashbuckler is, whilst stereotypically associated with the refined pirates of coastal domains, a natural fit for more civilized domains as well. Many swashbucklers make a successful living as mercenaries, professional duelists, and even as instructors in the arts of swordplay, a niche they compete for with bards of the College of Swords.
Thief
Undobutedly, this is the most common flavor of rogue in all the Realms of Dread. Wherever there is a reason to be good at stealing, these rogues will come forth.
Sorcerer
For all that mortals in the Realm of Dread fear and shun magic, it is a part of the world around them. Sorcerers arise all over, and even in lands where the practicing of studied arcana is a rarity, sorcerers may appear. Indeed, some DMs may decide that sorcerers are more common in lands where the practice of wizardry is a rarity, acting almost as a living pressure valve for the pent-up forces of magic.
Dragon Ancestry
Undoubtedly, these are the rarest of sorcerers in all of the Lands of Mist, for dragons do not, as a general rule, haunt these blighted realms. The most likely source of such sorcerers is the land of Darkon; beyond its place as a gatherhouse for the weird and occult forces, it is home to the Realm's single confirmed dragon, Ebb. Even if she is not willing to sully herself with dalliances with mere mortals, her presence could taint children conceived or born around her, infusing them with draconic power, if not warping them into calibans. Aside from Darkon, Nidalia may also have a small population of such sorcerers, the proverbial scions of Banemaw, crafted by the Dark Powers to mock Elena Faithhold. Beyond that, discuss with your DMs; some may be okay with your character unknowingly having outlander heritage, or being warped by sin into a more draconic form.
Favored Soul
These are the mystics and prophets of the Realm of Dread, beings born with magical powers in their soul that relate to no specific origins. Many of the most blessed Halan covens house Favored Souls, usually of the powers of Life or Nature, within their ranks, considering them the most blessed of the Weave-Workers. In Barovia, Favored Souls of Light may be born to the faithful of the Morninglord and carefully nurtured by the Dawnslayers. In some lands, these mortals represent the touch of the strangeness in the land itself; Favored Souls of Nature are extremely common in Verbrek, for example.
Shadow
More than any other of their kind, Shadow Sorcerers proliferate throughout the dark lands of the Realm of Dread, drawing their powers from the eerie forces that resonate in the land's corrupt soul and take form in the ever-writhing mists.
Storm
These sorcerers are most common in realms where violent storms wrack the skies. Though coastal and island domains are most iconic, any land where the heavens are violent can see the birth of these sorcerers. Barovia is a perfect example. If your DM agrees, Lamordia is another likely source for them, and may be one of the largest populations of them in the core.
Wild Magic
The purest form of magic, these are the definitive epitome of sorcerers in the Lands of Mist, born wherever the forces of the occult are strong enough to see them come into being. They proliferate in lands where raw magical energy abounds, such as Darkon, Hazlan and Vechor. At a DM's discretion, Lamordia may also be an ironically populous origin for these sorcerers, where they embody the wild magic that spills from the land even as Lamordians try so desperately to stifle it by denying its existence.
Warlock
Without a doubt, these are the most common form of arcanist in the Realm of Dread. Not all lands may host the resources to teach wizardry, and only those born to the magic may wield the power of the sorcerer, but anyone with the courage, desperation or malice to call out to the powers of the night may wield the power of the warlock.
Pact of the Fae
Four domains in particular give rise to these warlocks; the Shadow Rift, Rokushima Táiyoo, Forlorn, and Tepest. Those of the Shadow Rift are favored servants of the Sith and the Arak, swearing allegiance to fey-queens and faerie kings in exchange for their magic. Those of Forlorn view their pledges as being made to spirits of nature, oaths made to try and fight against the foul goblyns and worse that haunt their lands. Similarly, in Rokushima Táiyoo, mystics known as wu jen pledge themselves to the various spirits, or kami, and wield their magic in exchange. Warlocks of the Fae Pact are hated and reviled in Tepest, and those who live there often take up their oaths out of hatred, seeking to strike back against Wyan's inquisition for crimes committed against the once-innocent warlock.
Pact of the Fiend
In truth, a fiend-worshipper may hail from any domain in the Realm of Dread. That said, they are often found in lands where civilisation has flourished, allowing such foul lore to spread through backalleys and in secret societies.
Pact of the Great Old One
Warlocks of this stripe are a true oddity, even amongst their own kind. Though Gwydion, the foul monster that lies entombed at the bottom of the Shadow Rift, is the most obvious source for such pledges, the Great Old Ones exist beyond the mists, and who knows what strange powers may answer the call of one foolish enough to pledge their soul to the forces of the outer dark, wherever they may be?
Pact of the Undying
The Realm of Dread abounds in powerful undead horrors and beings that have cheated the cycle of life. From bokors in Souragne to Setites in the Amber Wastes to members of Darkon's Kargatane, these twisted necromantic warlocks abound throughout the domains.
Pact of the Undying Light
A truly rare and little understood pact, these warlocks seem to exist only to bedevil and plague the Darklords who serve as the damned rulers of the Realm of Dread. Where the undead proliferate, or the night is openly strong, these warlocks come into being, mortals selling their souls to burning powers in hopes of driving back the dark. The Dawnslayers of Barovia are home to many of these warlocks, but they may also be found combating the walking dead of Darkon, or battling the werewolves of Vebrek.
Wizard
Ironically, for all their power, wizards are amongst the rarest arcanists in the Lands of Mist. Most mortals fear their power, shunning it as something anathema to mortal souls and the balance of the world. Some domains avert this, to the point of being notorious for their mystical might; Darkon and Hazlan are the most famous examples of this. Others ignore magic, clad in the arrogance of their "civilized" status, such as Dementlieu and Paridon. In most domains, though, wizardry is a hidden art where it exists at all, passed down in small enclaves or, more likely, down a chain of masters and apprentices.
Abjurer
The irony of the populace's fear of magic is that they thusly shun this school, which serves so well for protecting people and nullifying baleful magic. Most abjurers arise in domains where the arcane is respected and accepted, such as Darkon and Hazlan, as they know that there is a real risk of facing rival spellcasters. Other abjurers are reared amongst theocratic sources, learning arcane magic to defend the faithful against evil spellcasters; this may be done out of benevolent intentions, such as amongst the Halans, or it may be done out of theocratic bigotry, such as amongst the Iron Faith of Nova Vaasa.
Conjurer
Much like necromancers and warlocks, conjurers arise all over the Realm of Dread, seeking to call upon powers from beyond the mists. With the maxim "do not call up that which thou cannot put down" alive and well within the Lands of Mist, it is rare that conjurers gather in great numbers, even in domains like Darkon or Hazlan. Most exist alone, operating in at most small cults or master-apprentice chains.
Diviner
These are the rarest of all wizards in the Realm of Dread, and many consider such talents to be restricted to the Vistani tasques. The mists cloud the unseen eye, impeding the efforts of these wizards to work their talents, and so it is a path usually seen as unreliable even amongst fellow wizards.
Enchanter
These wizards are quite common, flocking to more urbanized and civilized environments. With their subtle magics easily concealed and granting such opportunities, they make prey of their fellow man. Enchanters are especially common in Dementlieu, where they often operate in plain sight as "stage magicians".
Evoker
The fear of wizardry makes evokers a minority amongst their brethren; although their destructive powers are potentially devastating, they also excel at whipping superstitious peasants into frenzied mobs that blindly absorb fireballs and lightning bolts in their frightful dementia to destroy "the evil wizard". Evokers most commonly arise in domains that accept magic and recognize its talents for war; although popular in Darkon and Hazlan, check with your DM being playing an evoker from Falkovnia - most "textbook" depictions of this domain forbid such casters, given Drakov's bigotry against battlefield magic.
Illusionist
Brothers to enchanters in all manners that matter, illusionists are found throughout the Realms of Dread, using their powers to bewilder and beguile others. They are more isolationist than enchanters, using their powers to establish sequestered rural lairs for themselves.
Necromancer
The most common, notorious and feared of all wizards in the Realm of Dread, necromancers may be found anywhere, for their dark secrets have a peculiar knack for passing on from one master to the next.
Transmuter
The pursuit of alchemy is widespread in many domains, and even in ostensibly 'rational' domains such as Paridon, Mordent or perhaps even Lamordia, individuals may unknowingly practice arcane secrets in pursuit of the mysteries of shaping energy and matter.
Artificer
Associated with the most advanced and developed of magic-using domains, these are unquestionably the rarest of all wizards in the Realm of Dread. Discuss with your DM before using them. That said, there are two potential sources for them in the Core; Drakov canonically enslaves the wizards of Falkovnia and monomaniacally directs their power towards crafting enchanted arms and armor, and such a character, escaped from slavery, could be an artificer. Likewise, if a DM agrees, an artificer may arise from Lamordia, their vaunted "science" really nothing more than magic in a peculiar guise.
Bladesinger
Like artificers, these wizards are incredibly rare. As with the battleragers, bladesingers should not be restricted on racial grounds. Possible sources of bladesingers include the sylvan knights of Sithicus and the fey-trained warrior-wizards of the Shadow Rift, elite warmages of Darkon and Hazlani slave-masters.