A few Assist Trophy ideas.

Mii brigade

Considering Nintendo's active work on having these little avatars interact with games, I figure they would make a good assist trophy. They'll charge in as a group, slamming into everyone but the summoner as they run from one side of the screen, to the other. (falling off the edge like lemmings if the stage is suspended in the air.) naturally, they'll all be the Miis on the player's system.

Nayru, Din and Farore

They're a case of exactly what it says on the tin. Naturally they all spawn seperately, to make things fair. Anyhow, Nayru slows time for all players except the summoner, Din brings in a random weather effect(?) and Farore causes a random status effect on all players except the summoner (Like falling asleep, sprouting damaging flowers, exploding, etc.)

That's...All I got right now.

Rosalina - Super Mario Galaxy
Once summoned she surrounds the summoner in a magic bubble. The magic bubble also keeps the player from falling into pits, so basically, Rosalina is the Assist trophy version of the super star item.