Other than the Subspace Emissary Adventure mode?
Lessee... Probably a much bigger Classic mode, and maybe Break the Targets and Board the Platforms will make a return. I'm also hoping for a bigger Event Match mode, and the Multi-Man Melee. And Training Mode should come back with more options.
In terms of VS modes... more Special Melee modes, and maybe a Custom Melee mode (for example, you can have one player be giant, the other tiny, the other metal, and you invisible- like combining several Special Melee modes into one), and hopefully online mode.