Here's one:
The stage is rather roundish. Kinda like Brinstar Depths. Now, before you hit me with "BUH ISH BEN DUN!", here's where a new twist is.... Now, you take the stage, add gravity ALL AROUND. Maybe it could be a small Mario Galaxy promo level or something....
But how crazy would that be? I mean, you can get star finishes and side-wally kills like normal, but the floor kills would involve the same effort for kills too (so no more spike shots for those people who rely on them too much =P).
I was thinking of also a stage dealing with being in the bottled world on SMB2, where you could grab a key and piss off the "happy masks", but I never fully worked out that idea though.