I want to see Eggman make it in as a boss, using the Eggmobile to alternate between his different phases from the original Sonic game (wrecking ball, spike, fireballs, mines, you get it) but I have some ideas for Assist Trophies:
Tails: Tails flies above with his two tails and holds a remote control. He'll move around a RC-biplane (think the Tornado) around the stage that fires blasts at players. An alternate idea is he can also throw down Dummy Rings as bombs.
Amy Rose: Amy looks around and notices you. She'll scream delightfully and runs towards whoever summoned her. This has advantages and disadvantages. For instance, you slow down as Amy's hugging you. Yet, if anybody uses a projectile, Amy will use her Piko Piko Hammer to send it flying back with twice the speed. This also works if you're close enough to an opponent.
Silver the Hedgehog: He concentrates and then lifts up his hand as one of your opponents suddenly freezes wherever they are, and are drawn telepathically to come near Silver as they're pummeled in-air. He shouts "IT'S NO USE!" and then, with a flick of his hand, they fly out of his grasp. If they have enough knockback, this is a definitive KO move, like Lyn. However, with more than one opponent, you rely on luck to get Silver to stop the right opponent.
Team Chaotix: The 3 pop out and pose as they split up. Vector goes right and begins punching and biting with his jaws, Espio stays in his spot and throws ninja stars, and Charmy flies above and dives down occasionally. They re-group, do a pose again and disappear.
Knuckles was supposed to have one, but I couldn't think of anything too cool, he's got more playable character potential than Assist Trophy potential.