A new race: Vampires.

The 4th option doesn't entirely contradict, but I can't imagine a separate city with vampires on board. You could just as easily create a race of dragons (from small swamp dragons to huge magical creatures in the style of Terry Pratchett). And why split existing castles when, if you're thinking of making a mod, it's best to come up with something new? Although, with a clever approach... I don't rule anything out.