Frostwraith
The Demon King
With Fox confirmed to return, we will surely see Star Fox stages in this new game. The 3D effect sounds neat for more dynamic stages like Brawl's Lylat Cruise. I wouldn't be surprised if we got a stage based on Star Fox 64 on the 3DS thanks to the remake, much like how Gerudo Valley has been confirmed to appear on the 3DS version too.The first thing that comes to my mind for a 3DS stage is a Star Fox stage. Asteroids and all that pretty space stuff.
Anything could work, but obviously they're probably going to keep the 3D effect in mind with all these stages, so I have a feeling that they might want to use concepts we've never seen in 3D before. Think about Mr. Game and Watch's Flat Zone on the 3DS, ya know?
The Flat Zone stage could have the 3D effect deliberately off to convey the 2D feeling of G&W games.