Where did all the warlocks go to?

[TheArcaneWeirdo]

Looking elsewhere, I feel like each of the Three Hags would have their own coven and they'd alternate between working together to do things none of them can do on their own and viciously backstabbing each other when they think they can get away with it.

perfect.
Also, what about 'Mother's Minders' from VRGtR? I think it's a nice twist, a warlock that has a warm, familial relationship with a thoroughly evil patron, that grew up in a society where this wasn't only acceptable but idealised, that genuinely doesn't get what's weird about it.
(I love how - like, with other domains, you have to come up with a really complex theory to explain the edition gaps. The Three Hags are just wacky enough for me to accept that Lorinda just woke up one day and decided to trap her sisters in a cauldron, turn the inquisition into axolotls and start a cult in Viktal.)

[Drinnik Shoehorn]

I've been thinking about it and I think there are a few options for 5e Ravenloft warlocks (using the old Canon, not VRGtR):

Archfey: Loht from the Shadowrift, any one of the 3 Hags (or maybe them collectively), Blackroot the evil Treant, Addar the Shadow Unicorn

Celestial:

VIEW CONTENT:

Isolde is the only known celestial in Ravenloft, and I can't see her becoming a patron to anyone outside the Carnival.

Fiend: Take your pick. Inajira would definitely be making deals left right and centre. The Gentleman Caller might as part of his master plan to escape. If you ignore the fiction from the BoS, Drigor is still in the Mists, and The Black Duke would love to expand his cult, not to mention all the fiends in Children of the Night: Demons. Stryx could fit here too, as she's a Night Hag, the pacts could just be one way of her fuelling the Rift Spanner.

Great Old One: The only real option is Gwydion in the Shadow Rift, unless there's someone I'm forgetting. Anton Misroi could maybe fill this role, as hes evolved into something more than a common Zombie Lord. Inza might work too, as we aren't sure what she is post-Spectre.

Hexblade: Ebonbane is, again, the only real choice. The Sword of Kas, maybe, if you're playing in a pre-Die, Vecna, Die! setting.

Undead/Undying: Lyssa von Zarovich, for reasons stated above. Simon Audaire, Ankhtepot, Anton Misroi, Death, Lord Soth pre-Spectre, Pieter van Riese could become a patron to lost sailors.

I think the biggest question with Patrons is do they have the power to fuel the pact? You aren't expecting the PCs to get to level 20, but in theory could the Patron supply that power? The Patron doesn't necessarily have to be level 20, but storywise, can they fuel that power?

[The Dark]

Great Old One: The only real option is Gwydion in the Shadow Rift, unless there's someone I'm forgetting. Anton Misroi could maybe fill this role, as hes evolved into something more than a common Zombie Lord. Inza might work too, as we aren't sure what she is post-Spectre.

The God-Brain seems like another option here.

One scenario that comes to mind but I'm not sure how to categorize would be taking the Seekers of the Spark from Lamordia and changing them to a cult of warlocks to Adam (actually powered by the Dark Powers because that's not Adam's thing). It twists the "no magic in Mordentshire" thing and lets the golems reject the science that forced them into being.

[TheArcaneWeirdo]

Great Old One: The only real option is Gwydion in the Shadow Rift, unless there's someone I'm forgetting. Anton Misroi could maybe fill this role, as hes evolved into something more than a common Zombie Lord. Inza might work too, as we aren't sure what she is post-Spectre.

One scenario that comes to mind but I'm not sure how to categorize would be taking the Seekers of the Spark from Lamordia and changing them to a cult of warlocks to Adam (actually powered by the Dark Powers because that's not Adam's thing). It twists the "no magic in Mordentshire" thing and lets the golems reject the science that forced them into being.

Cool! Now I want a story about it.

[Baron Von Stanton]

Celestial:

What if Isolde needed an agent or representative to go somewhere where the Carnival can not go?