I'll make a shot-in-the-dark prediction about the Donkey Kong stage. Don't know if it'll be for the 3DS or WiiU version, but it feels right.
Basically, a mine cart stage, from World 4 of Returns. It'd be called "Mole Train" and based on that world's boss level. Given that it'd be a train level, I guess it'd be better suited for the WiiU version, since the 3DS version already has Spirit Tracks.
Also worth noting that the remix for the Rocket Barrel theme is in one of the games. We heard it in the Smash Direct back in April.
This mine cart stage could feature the falling diamond-encrusted ceilings, the moles throwing bombs at the track, the giant bat that chases you and unleashes shockwaves, and more. You could have the shantytown in the background, the waterfalls, the over-sized mushrooms, etc. It'd be a great way to represent various backgrounds from Returns and the mine cart concept from the Country games in general.