personally i think the argument "castlevania has never been about story therefor an rpg wouldn't work" is invalid. its true that in the beginning, castlevania didn't focus on story, but that is no longer the case & hasn't been for quite some time. the Dracula/vampire mythos is what keeps many of us coming back for more, it coincides with the gameplay, and the series has had more story since the mid-late 90s with symphony, CV64, etc. the last early-90s era cv game we got was Adventure ReBirth, that games plot can literally be summarized in a short sentence, as with all the other early 90s games.
DragonSlayr81 mentioned a while back about konami capitalizing on the Dracula mythos by including elements from real life, his brothers, the Order of the Dragon, the political struggles of ancient Romania, etc. perfect opportunity for that to be realized in an RPG, is it not? its somewhat difficult to cover those interesting aspects in a action-platformer. consider a reboot that used the "real" Vlad Tepes (no mathias, no Gabriel, just original Vlad). the game uses elements from real history while blending in elements from the games (monsters, Belmont Warlord Chromosones, etc) the climax will have Vlad fully transformed into Dracula, and the Belmont hero Trevor or whoever faces him in an epic showdown, Cloud vs Sephiroth style. it could work, just gotta be open-minded. perhaps it could be a final fantasy tactics/tactics ogre style game