Hrm...let's see...
"Duplicate" characters like Paper Mario and Y/WW Link (I don't care if they can have unique movesets, the joy of SSB is variety, and guys like this take away from that)
Any Square-Enix character NOT named Geno or Black Mage/Moogle (I don't think those last two have much of a chance, but I wouldn't mind seeing them)
Waluigi (Would be a collosal waste of game space. Really, he's famous, but he was born to be filler, and has nothing to offer)
Daisy (Well, at least she used to be unique. But, like Waluigi, she's nothing but filler now)
Pac-Man (Come on. He was in that crappy Mario Kart arcade game and nothing else. If he's chosen over Megaman and Bomberman I'm killing a small animal...Okay maybe not, but still.)
Ashley (I like Ashley as a character, but Jimmy T. or Mona would be much better choices. Especially Jimmy T. *imagines a FS as a giant wave of dancing cats*... :3)
Any other Sonic/Metal Gear character (It's okay if they're ATs though. It hurts me to say this, because Tails may be my favorite VG character of all time)
Blaziken/Plusle-Minun/Raichu/Any other Pokemon not named Mewtwo, Jigglypuff, or Lucario (Simply because I can't see it working out. They'll probably all go the way of Gardevoir and Meowth and be Pokeball'd.)
More than 2 FE characters (I admit, this one is 100% opinion, but I really don't care about Fire Emblem, and all more FE characters will give me is more characters to ignore. Plus I'm still bitter about everyone and their friggin' dog playing as Marth and Roy when I played Melee with friends and such)