I think it looks great, very doable. I like how they look like the original graphic style but with added detail and frames. This way when we bring in straight gfx rips (like fleaman) they will fit in with your graphics.
On to the list with comments:
Axe Knight
* Axe knight, even a Disc Knight that shares most of the same graphics)
Bat
* Red ones fly from off the screen at you, Blue hang from ceilings and attack when triggered
Bone Pillar (shoots fireballs in both directions, non-moving, as well as possibly a walking variety of bone pillar)
* mm, I think the stationary one only and save the frames for another enemy in the bank. It's one of the few enemies that takes up hardly any room.
Crow
* Sits on a perch, then attacks, I like the idea of a swarm of these when you walk into a room.
Dhuron
* Classic CV3 but with an alternate attack I'm thinking, shame though cause I like your big one a lot.
Fireman
Fishman
* Your fishman so far is great. I see yellow ones, green ones, red ones

Fleaman
* Fleaman, then Red Rippers that use the same graphics and throw red daggers at you. (possible special sub-weapon he drops)
Floating Eye
* Yes maybe even a rolling eye from the GameBoy games.
Flying Skull
Ghost
* I want to bring the ghosts back from CV1 maybe as a familiar or an enemy
Giant Spider
* maybe even an arachnid lady
Giant Frog
* love the frogs, I considered a frog familiar even.
Giant Leech (may latch onto Simon and suck life until shaken off)
* this is possible (add the leaches from CV2 but have them stick love it)
Harpy
* thought the cv2 graphics could be improved.
Man-Eating Plant
* are these the same ones that you have in there now?
Marionette
* Like the idea once defeated they might drop the doll subweapon
Medusa Head
* The ones I ripped from Vampire Killer I like
Mudman
Owl
Skeleton (bone throwing and non-bone throwing)
Skeleton (Whip Skeleton)
Skeleton (Bone Scimitar)
* always need these, maybe get rid of the whip and sword and come up with unique skels
* some Skels I considered were Farmer skel (drops garlic SubW), Archer skel, head chasing Yorick)
Slime
* Slime will be in there but I ripped their graphics from Vampire Killer, in that game they would fall out of candles randomly, i think I will incorporate them that way.
Spear Knight
* Yes these but also some knights with original weapons
Vulture (in place of the classically used Eagle, I've always thought this series needed vultures instead of eagles)
* Sure, I also want to paint pictures with enemies, like have the vultures eating some bones before they attack you.
Werewolf
* Already think yours is great, maybe give him a rare tough fight appearance
Wolf/Panther/or Hellhound (don't want too many similar enemies, but while wolves are classic horror, the panther has been used more for the Castlevania series, not sure yet)
Zombie (thinking of having both male and female ones, maybe even children zombies)
* yes zombies can be back in full force love this idea.
Other enemies.
Farmer Skels
Succubus - blows heart kisses at you
Ghoul - from CV1
Treeman - other tall enemies like Iron Golem that are one room battles
poltergeist aka Flying books- use these in a library scene
Mummies
Giant Skulls
Crypt Keepers - lantern weilding
I can't think of any right now, but I would like to see some enemies from Symphony brought into this mixed in with classic ones from Vampire killer/Castlevania