I'd love to see a stage based on Dragon Roost Island. Maybe Valoo could shoot fire at people occasionally, or if you hit him you can get a scale which would allow you to fly for a short period of time. I'd also like to see them bring back that stage from Melee where Kraid rotated the whole thing. That was awesome! maybe update it so there's more platforms or a way or two through the center, or even a second or third circle for Kraid to hit. A Mario Galaxy level would be cool as well. You could make it big like New Pork City, with all of the platforms being rotating spheres which you fall off of if you stand still. You could have a fire one with a hippo that attacks you if you land in front of him, an ice one that's slippery, a big grassy one, and a desert one with platforms that sink as you step on them and maybe even pop out the other side.
A bombchu would make an interesting item. I'd rather see them get rid of some of the old items though, like the cracker launcher. And change the smash ball mechanics so that it only shows up under certain conditions rather than randomly.

I wanna slam bowser's face into the ground. :3