hmm where to start:
n64 vania's: Time to finish the game, better camera, Coller as a character, he was one of the more interesting characters the series had in a while IMO.
COTM: I dunno why but deaths seems like a completely pushed to the side character here, if I remember correctly he's stuffed away in a basement corner somewhere and he's (optional?), memory might be deceiving me but still, they could have done more with him. Also no extra character is a real miss here, imagine the fun of the throne room battle being swapped :O
AOS: like stated before: more interaction with Death, Julius mode with a tiny bit of story.
DOS: could easily have been one of the most memorable games had they: not reused alucard's sprite and actually made a 'new' alucard, what was mentioned before about Death's role in the game, left out the anime art, burning village/city stage as intro!
OOE: less linear stages, their concept was good but some were too one dimensional, adding more depths to these would have greatly improved the game. More info about Albus's quest.
LOS: less flat enemies, was it just me or did it feel like every section of the game had like 2/3 enemies and after you finished they vanished? had they mixed them more and made better use of them it would have been less repetitive fighting the same enemies over and over until they became extinct.