First of all, I hope they restore the watered down mess that was the single-player mode in Brawl. It seems like the focus was put all on SSE, because if you look at the other single-player modes they all reverted back to the SSB days - no, actually worse than the SSB days. For example, stages were repeated in the modes, like you'd fight a giant twice in a single play through a mode. All of the modes felt watered down, though, that's just one example.
I don't want another SSE. I appreciate that Sakurai was trying to do something different, but I really preferred the Adventure mode of Melee, and would prefer to see an even better version in SSB4, taking you through all of the worlds of the characters added in the game.
Alternate costumes, like they did with Wario in Brawl. Mario should have Dr. Mario as an alternative costume, Pikachu could have Pichu and Raichu as alternative costumes, Pokemon Trainer could have alternative costumes (at the very least add Leaf, the FR/LG female trainer that you could choose instead of Red) Isaac could have Matthew and Felix as an alternative costume, Marth should have Roy and *another character* as an alternative costume, etc. I guess it all really depends on what extent the alternative costumes could be taken to.