I want the blue turtle shell from New Super Mario Bros. The odds are against it since they didn't even have it in NSMBW, but I can hope, can't I? It just has so many applications. Imagine how you road koopa shells through water in the first game, except with this you are the koopa shell. Plus, shaking the Wii remote to send yourself flying into an enemy instead of just tossing a shell. That's verging a little close to Sonic, but it would be fun. You could still slide around if you ran for long enough, and this time it would be in 3D.
Also, I'd like to see WiiMotion + capabilities. Not as a necessity, but as an option. Things like flying, rolling around on balls, surfing, they would only be increased. As for a big open world instead of just choosing levels like in Mario 64, I hadn't thought about it but that would be nice.