Other people have said it before.
I want a ranked and unranked playlists.
Ranked would be stock with a time limit, 1v1 or 2v2 no items, and only matched against and with people of similar skill or 'rank'.
Unranked would be whatever they want, time, stock, 3 player ffa is a particular favorite of mine, items galore, Basic Brawl etc.
I want a romantic subplot between Marth and Zelda (obviously not going to happen).
I want to lol all over their roflcopters.
I want modding support (not gonna happen, hopefully they just don't add unnecessary layers of security).
Custom skins doesn't seem too far of a stretch (lel, get it? Skin? Stretch? Canniba-I mean-uh).
I want a model poser for screenshots or video (perhaps coming in with the modding.)
I want a game that has a low but difficult to master skill ceiling (because I'm bad and do far better in Brawl because of that yet am still a ways away from tourney material). I don't know how that would work, but it would be interesting.
I want lots of random quirks that develop and significantly impact the metagame over an extended period of time, perhaps in the DLC or even. . .
. . . I want tons of custom options (modifying physics engine), maybe this will also come along with modding, as in they just minimize the security and allow people to have at that stuff, kinda like Melee's Master Debug Menu.
I want the obvious stuff: More characters, super mega ultra HD graphics with 4000p, better stage builder, I liked the Brawl sticker system so something like that in the 3DS/singleplayer, the continuation of the best video game soundtrack of all time (obviously arguable).
No expectations, just hope, and my hope can never be crushed.