Rob can get the boot. He's lame, even if he fights well.
Luigi needs his moves re-examined or else he needs the boot.
I'd say keep Wolf and Ditch falco, as he mirrors fox more than wolf does and the game could use more villains.
Ditch meta-knight. He's cool and a total badass, but he's totally broken and no oe wants to see him again except as maybe an assist or something.
Ditch the jigglypuff. It's a troll clone of kirby.
I would be fine with swapping out lucario for zoroark, but I'd definitely prefer Mewtwo back. Especially if they fix his forward special to be a real reflect or counter.
I like Ike, but it would make sense if they either replace him with someone from the new FE or put the new FE person in addition to Ike and Marth.
Knuckles would be a viable fighter from Sonic, but everyone else needs the stay the hell away. Tales wouldn't even make a decent assist everyone else gives me an aneurism thinking about them.
Get rid of toon link. Smaller, weaker, slightly faster, and totally unnecessary link. I could see them updating some of his moves.
Pit- update his moves, especially given his recent game and his spam abuse in Brawl. And please, change his voice acting. He's annoying as hell on so many levels.
Pokemon trainer- they should add in a trainer that uses other starter pokemon, like totodile, bayleef, and Quilava. But really what they need to do is make an entire pokemon fighting game altogether based on the brawl battle engine. You'd necessarily have a lot of overlap for moves and stuff, but that's okay, cuz then it comes down to mastering your favorites or using the right pokemon with the right move combination for ya.
Mario- update his moves, he's got plenty of new stuff to choose from all his newer games. FLOOD is just lame.
Dry Bones- I'd like to see him added but I see it as highly unlikely and I'm not really sure how he'd fill out him moveset anyhow.
Isaac- I'd really love to see him in there and after the fanrage of him being a really lame assist trophy last game will hopefully push the developers to really include him into this one. They'd probably opt to put Matthew in instead though because he's the main character in the newest game. He's got all sorts of psynergy available as special attacks for him, and summons or Meggido unleash as his ultimate. He could maybe do something similar to olimar in that he can either summon or switch out attached djinn to alter his special attacks. Move might actually be a pretty awesome move to keep people from recovering to the stage. Pound, force, and other 'utility' psynergy and attacks could be some viable specials. They'd really have to explain really hard to me any reason for Isaac or Matthew to not be included as a playable character.