I also like the
Black Dragon, although I agree with those who think its status as a spelljammer should be kept hidden. All PCs would need to learn is that it desperately wants to
travel -- maybe just through the sky, maybe to the Moon, maybe to other planes or weird Lovecraftian dimensions --
without necessarily tying Ravenloft to a setting full of hippo-headed blast addicts and giant space hamsters.

An oblique reference to SJ is fine -- heck, Mystara can be every bit as wonky, and Meredoth still fits Ravenloft -- but a blatant one is going too far, IMO.
The 'Dangerous Liaisons' darklords are interesting, but I'm not convinced they wouldn't work better as non-lord villains than domain-bound ones. I can easily picture those two as a couple of Ivana's ermordenung, taking out their own frustrations on an Innocent for kicks and giggles. If you want to keep the erotic side to their relationship, we can always do what TSR's code of standards never let
their writers say outright, and have the duo indulge in sexual games that don't require direct skin-to-skin contact.
The Romeo-and-Juliet story, to be honest, seems like it'd be a great short story, but only a so-so domain. Nobles with nasty habits are so common in the game-setting, it's hard to say how much the nosferatu-wannabe is going to stand out -- assuming players don't get her confused with Talena von Zarovich -- or if the heroes would ever delve into her history, except possibly at the very end of a campaign-arc ending in Guendalina's death.
Chiara's an interesting villain, and I'd have to rate this one near the top of the choices presented so far. I do have to wonder how Chiara feels about other
women, however, as IMO an awful lot of her isolation and bitterness towards humanity would be ameliorated if she simply had a BFF in her life. So why doesn't she find one, if she craves the company of peers who won't just treat her as a political tool or sexual prize?
As for the elf, she seems too out-of-touch to rate darklordship. If she's truly incapable of comprehending death, and drains life from others due to a magical affliction she never asked for, then damning her to a domain for killing people is like damning a three-year-old for innocently shoving a playmate into the pool to drown. She's also far too similar to the Lady of Ravens.
"Who [u]cares[/u] what the Dark Powers are? They're [i]bastards![/i] That's all I need to know of them." -- Crow