If they set a Castlevania in the 1930'3 I would totally insist on Slenderman style wierdos be in it. And some more Lovecraftian monsters too.
I always though a proper remake of CV1 should have added two bosses, The Creature from the Black Lagoon and Wolf-man. Only then would the game's nod to classic horror monsters be truly complete.
Oh and for the sake of the argument I'd reorder the bosses like this:
Stage 0 - Wolf-man (I always thought they should expand the main courtyard into it's own stage.)
Stage 1 - Phantom Bat
Stage 2 - Medusa
Stage 3 - The Mummies
Stage 4 - The Creature (from the Black Lagoon)
Stage 5 - The Monster & Igor
Stage 6 - Death (This would be the Clocktower as it's own expanded stage)
Stage 7 - Dracula (And this would be the Castle Keep from Vampire Killer which is bigger than in CV1)
Wolf-man would have to be rather simple as a first stage boss, and I'd make Phantom Bat harder to compensate.
The Creature, now taking up residence in the Monster's old room, could have water added to the bottom of said room so he can jump in and out of it constantly. That would make him harder to hit and keep the player guessing. He would be harder than the Mummies, but not as insane as The Monster with an immortal Igor.