For Characters:
50 characters! (More if you want to count the transformations.) Have Roy and Mewtwo return, along with everyone in Brawl, except Snake. Sorry. Dr. Mario should be an alternate costume to Mario.
I would love to see Takamaru, Laughing Dog, Sami, Kumatora, Krystal, K. Rool, Bowser Jr., Little Mac, Ridley, Medusa (But I guess, i'll take Palutena), Saki, and Waluigi (It just doesn't feel complete. We need that Mario/Luigi/Wario/Waluigi fight.

) There's more but I can't think of them right now. Hehe.
For Stages:
I would to see the Luigi's Mansion stage re-done, I was disppointed with it. :x Bowser's Castle is the only other stage I would want.
I want stages from other games, that aren't repesented by playable characters. There is so many possibilities, and I just really look forward to them. That and the option to turn off hazards on stages.
For Trophies, and Assist Trophies/Pokeballs:
Basically what IntelliHeath said, I agree with everything.
For Single-Player:
I would love the return of character specfic Break The Targets, Board The Platforms, Race To The Finish. All that, everyone mode that was given in Brawl. Maybe give us a random generated Adventure Mode, that's of course way more fleshed out. I'm not sure, but yeah.. I think on single-player right now. XP
For Online:
Give us the connection we deserve! Give us more options, to create our own games. Tournament mode, Lobbies, etc. Oh, and a better Spectator mode.
For Items:
Every item that has appeared in smash, to return! Give me a blue shell as well!
Miscellaneous:
I would love two more color options to choose from, and everyone gets the same amount equally. Poor Pikachu. I would love to see alternate outfits for characters, but that won't happen. Dry Bowser!
I probably missed out on a lot of things, but oh well. I'm done. Lol.