I expect a 3rd person 3D Metroid. Yeah, you heard me. Not that it won't have a 1st person mode. I think they'll try toggling between the 2 in the next game.
Mach Rider. Aim with the Wii remote. I expect it to be very fast-paced and arcade like.
3D Kid Icarus. That one is definitely (finally) coming.
A 2D Mario game. Nah, that'll never happen on Wii.
A true Super Mario RPG sequel. I'm really stretching with this one, but I think it could happen. The demand is there. With Square Enix getting so chummy with Nintendo lately, it's looking at least possible now.
On DS, I hope for (but don't expect) Super Wario Bros. Waluigi is finally playable in a non-Mario sports game and his character is finally justified.