There are many things that could be said, but I wonder how many could work. The problem is: what story could work so that a determined evil force or person would subject itself on a struggle to help Dracula? Actually, I don't think that an evil being is necessary, after all, Castlevania is about fighting evil, this evil having any face it might have.
If what you meant wasn't about making a plot, just having other enemies instead, than it's easier. A good example of unexplored enemy is the baobhan sith (just ask wikipedia). However, I think that making up horrors like in that cool artwork from Ayami Kojima (scans in Vampire Killer) would be more than enough.
But if you really meant to suggest a plot using an unexplored character, than I'm going completely off your expectation and say that Kaguya could work somehow. Kaguya wasn't evil, but since there are good creatures at Dracula's side, who knows? Also, she require impossible requests for the protagonist to accomplish (only they wouldn't be really impossible).
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