Forgot to add:
-a fourth DS Metroidvania. I think Iga had really refined the formula in OOE, and it would've been nice to have him take it a step further on the same platform.
-a port of Castlevania the Arcade for Wii
Oh, and a DS Wayforward Castlevania 5 would've been awesome with multiple characters (starting with a new Belmont), branching paths, and multiple endings. A true Dracula's Curse sequel, in short.
An arcade castlevania game with the similar vein as Strider Hiryu 2: 2d sprites in 3d backgrounds, a simple linear game.
I love Strider 2 and I didn't realize a Vania like this would've been great until you brought it up.