I don't think I want to see a Castlevania fighting game, especially not again, but I guess if it had to happen, I think Maxim Kischine's ninja-like skill set would add an interesting flavor to the hypothetical game. Maybe he could also go into possessed mode when certain conditions are met, granting him different or enhanced abilities, or something.
EDIT:
I was about to say that a Succubus would be a good idea, but I think it would be more interesting to include Annette Renard as L. Vampire. Or... have both of them.
ALSO!
I could see Blackmore having a lot of cool moves. Might want to diminish the size of his shadow or only have it come out at certain times, though. His room would also make for an awesome battle arena.