CV1 - I would make a definitive version that combines all the stages from all the different tellings of the CV1 story into one uber remake
(I'm already doing this in 8-bit! ^_^)
CV2 - This NEEDS to be remake for the love of all that is holy! Actually, there is a ROM hack in production that aims to improve the original. If it were done as a traditional game it should be modeled after CV3, but if not...
1 ) Full set of bosses that function as they do in every other CV. None of this oh-here-I-am-in-this-room-but-I-don't-feel-like-even-locking-you-in-this-time-cause-I'm-just-too-lazy-bullshit! I wanna see some bosses with at least the complexity and aggression of CV1 if not CV3.
2 ) Screw the stakes, the 5 most powerful bosses should just give you the Dracula parts for defeating them.
3 ) I demand an in-game map!
3 ) GP instead of hearts for money.
4 ) Real equipment instead of equipment style sub-weapons. Sub-weapons should work like in all classic CV games, and I demand open flames from which to get hearts and sub-weapons.
5 ) Add regular vampire enemies to give the stakes a real purpose instead of a bullshit one.
6 ) Rewritten script. The towns people can still lie about stuff to confuse the player, but I demand proper English sentences and at least one person per clue that is telling the truth. While were at it, the secret clues in the mansions should be available to view from the game menu after they are acquired.
7) New rule: all fake blocks must get permanently destroyed when struck by any sub weapon.
8 ) Holy Water must produce the Sacred Flame effect upon impact and Sacred Flame should be redesigned or replaced by something useful like a healing herb. Instead of 3 versions of the knife, make them upgrades and add the Axe and the Cross in their places. Also, add Stop Watch.
9 ) New graphics for the mansions so they actually look like mansions instead of dungeons and a different tile set for each mansion.
10 ) Castlevania must rise again after Simon collects the Dracula parts instead of fighting his ghost right then and there. (Even if the castle is only 3 or 4 stages.)
11 ) Dracula's ghost must not be a pansy little bitch and must have at least 2 forms of epic-ness.
12 ) Almost forgot! Password system based on CV3/SCV4 versions.
CV3 - Everything Archangel said, plus more bosses and music to replace duplicates. Allowing all the helpers to join would probably be easy without revising the path system. You can already encounter Grant and then Sypha on one path, so just add one additional path choice that lets you pick between the Ghost Ship and Alucard's cave and you could get them all. Maybe add a second choice to go back to the Ghost Ship after getting Alucard to complete the loop back.