I think keys could work in a 3D castlevania. Depends on how many keys we're talking about though xP If we assume the next-gen Castlevania would be like Simon's Quest but in 3D (which would kick ass, by the way) it could work, if the key-finding phase was only in mansions, for example.
Another oppinion of mine for the next-gen Castlevania is to get rid of the whole leveling-up system. Almost every game as a leveling system these days and it's getting tiring. Plus it's starting to feel kinda useless and unessesairy. Instead you can get stronger only by finding new weapons, equipments and skills (like the Metroid series).