Ness, Lucas, King Dedede, and Pit's need more stars/minions. Actually, give either Ness or Lucas PK Rockin'/Love. I don't care which. <_<
Kirby's needs more knockback. It can't even KO until ~100% iirc.
Falco, Wolf, and Toon Link's need their own.
Fox, Wario, and Sonic's need to have less time and power.
Pikachu and Diddy Kong's need to be easier to control.
Bowser's needs to be invincible or less vulnerable to slowdown from attacks.
Peach's needs to cause more damage and have a stronger sleep effect.
Ice Climbers' needs to damage you even if you're in the air/on the ledge.
Jigglypuff's needs to be totally redone imo.
Zero Suit Samus's needs more range and power.
Donkey Kong's needs to be easier to use, if not totally freed from user input.
Link and Ike's need to have less knockback imo.
Captain Falcon's shouldn't cut to that cinematic.
Pokemon Trainer's shouldn't focus the screen on the area immediately in front of the Pokemon. The move itself should be harder to escape from once caught in it as well.
Olimar's should either do less damage or not crash the ship for what's pretty much an auto-KO if you're hit by it.
R.O.B.'s...needs something. IDK what. But it's so...boring.
Mr. Game & Watch's needs to shrink just a tiny bit.
...I think I just went way, WAY overboard with this list. <_<