I don't know... I'm already thinking how awesome stuff would look on the 3DS, but it doesn't need to be a 3d action game. They could go the way of The DraculaX Chronicles but only with a slightly more 'overhead' view (like Kirby 64 where you can see more background even though it's a sidescroller).
Perhaps I'm a bit spoiled since I'm playing the 3DS right now, but having any kind of Castlevania game (2D platformer, 3D action, 2.5d platformer) would look pretty great what with the higher resolution top & bottom screens, and of course, the 3D slider. I've been playing Steel Diver and that game is a 2D platformer with 3D graphics and at times it feels like I'm looking inside an aquarium and piloting the subs as such, within. Can you imagine Castlevania with enemies who come from the foreground, or perhaps with rotating sections (still platforming, but now you can really see if you're on a rotating tower, etc. as you platform up)? I think it would be really cool in any instance of the genre.