There's too many dang characters that could be good AT's.
Off the top of my head...
Toad (if he's not playable or Peach's personal meat shield), 8-bit Bowser (from SMB), Wart, Bowser's Children from SMB3, Baby Mario, Kamek, a herd of Yoshi (I'm not sure how it'd work), Donkey Kong Jr, Cranky Kong, Funky Kong, The Great Fairy, Midna, Epona (for the lulz), Wolf Link, the Wind Gods from the Wind Waker, Fierce Deity Link (YESSS!!!), Kraid, SA-X, Dark Samus, a group of Space Pirates, the Hunters from Metroid Prime Hunters, Ninten, Master Barf (best. boss. EVER!), Porky, Poo, Dr. Stewart, Polar Bears (c'mon, they disappeared from Brawl!), random G&W characters/things, the Black Knight, Sothe, Maichiah, Eirika, Ephraim, Hector, Medusa, Revolver Ocelot, Liquid Snake, (Ninja) Raiden, The Boss, Colonial Volgin (probably too violent, sadist, and overall evil for smash), Drebin's tank, Solidus Snake, Tails, Knuckles, Dr. Robotinik (down with Eggman!), Metal Sonic (YEAH!), Dr. Willy, Gutsman, Electman, PharohMan, Bass, Roll, Tom Nook (he's totally evil-with an extra e!), Miis, .... too many to name!