I would seriously love to have Roy back. I miss him in the old game. I wish Microsoft didn't own Halo, then Master Chief would be in.
I would want to see Krystal from the StarFox series, Tails and Knuckles from the Sonic the hedgehog series, Replace Lyn as an assist trophy to a playable character, return Mewtwo and Pichu back into the game, and add Roy back into the game as well...I can't beleieve Viewtiful Joe wasn't in brawl, if they do have an espansion pack or a DLC, Viewtiful Joe should definitely be a character...
I would lovee to play as toad =] And to kick slippy's ass .. I would like a mii feature too ! That would be cool! =]
I would to see Mudkip from Pokeman. That would be awesome, and the Mii feature that Nemi said would be a nice addition.
Come to think of it, I would love to have some characters from a manga series. Such as maybe Goku from DBZ, or Sasuke from Naruto. I would also love to see Pac-Man in there. That would be awesome.
Seriously, I would want some more people from the Metal Gear Series.... just not Raiden. Ugh, stupid sissy boy. Also some more Fire Emblem characters would be cool. Personally what I would like is more costumes for everyone. Having different colors of the same costume is still bland. It would be nice to see some other outfits for other characters. Oh and like someone earlier said, ability to play custom maps online... That was stupid to include if it wasn't going to be online accessible. Lastly: Remove Pit... He's really god damned cheap. (I play as snake)