Character speculation has gotten bland/boring since this year, so I'm more interested in whatever else we'll be getting in future updates.
Aside from the easy to guess returning modes, I would like it if they introduced a new/updated Boss Rush, with returning bosses from Brawl/Smash 4 (a few of them like Petey, Rayquaza, Metal Face already have in-game models to work as a base). I'd find that a neater way to represent Porky, assuming the bosses would be updated the same way Galleom was with some new attacks. I'd also vouch for Ridley on Pyrosphere to return in the same way Ganondorf can still fight Ganon.
The other obvious choice would be a mode for that one spoiler fight in World of Light. Maybe even make it co-op, or add some variations ala Mob Smash/All-Star.