Hopefully NOT another Sunshine or Galaxy experience.
Let's get back to basics. Give us a full 3D Mario (3D World is nice, good game, fun, but enough with floating micro-levels thing). Give us a full, unadulterated romp in the Mushroom Kingdom, in 3D, in HD, for once. We've never gotten it. We've gotten magic paintings, an island, and space/tiny planetoids. Let's get a full blown massive Mushroom Kingdom, in all of it's magical wackiness, going on. THAT is the 3D Mario I want to play next.
No gimmicks, no new "theme", just Mario, Mushroom Kingdom, Bowser, cool powers (old and new), and a LOT of classic Mario gameplay.