Recently Nintendo added a lightgun arcade game called Mad Dog McCree on the 3DS eShop, and it got me thinking "What if other lightgun style arcade games could be ported onto the 3DS?"
So I think Castlevania The Arcade could be really awesome on the 3DS. There's already shooters that use the touch screen to control the targeting reticule, making the top screen where the action is, and they could make that action into 3D and it'd look awesome because it's from a first person perspective.