Hey let's think up new Assist Trophy ideas!
Big the Cat: He sits down on stage and starts casting his lure out, think a much longer range Isabelle Side Special. If he catches a fighter he'll reel em in and throw em away. thrown fighters deal damage to other fighters they fly into. Occasionally Big will catch an actual fish, he will hold it above his head, these fish can be eaten by players to restore a bit of health.
Fi: Fi dances across the stage doing kicks and transforming into a sword to swipe at fighters. she can be attacked and defeated in fights.
Chaos: Chaos will throw out his long stretching arm to attack fighters, spinning around, turning into a large ball of water that bounces players off its surface and can transform into a puddle to trap players while moving around as well. On rare occasions he will transform into Perfect Chaos and perform massive stage covering tidal waves and energy beams and tornados doing widespread absurd damage, this can be stopped if a point on his head is damaged enough.
Penn: He flies around throwing newspapers at everyone, he will also swipe at fighters with his large wings and bite with his beak.
Lanky Kong: He can inflate himself like a balloon and act as a platform for fighters. Attacking him in this state will end his inflation early, Lanky will also stretch out his arms to attack fighters and move around to shoot at fighters with his grape blowgun. If he KOs a fighter he will do a little dance.
Magda: flowers will sprout around her, if any fighters step in these flowers or attack them she will violently attack the fighter(s) with frightening speed, she can very easily KO most fighters.
Heartless: Heartless will spawn all over the stage, attacking anyone that comes near them, the heartless that spawn are Shadows, Darkballs, Large Bodies, Green Requiems, Hot Rods, Parasol Beauties and Invisibles. Shadows Darkballs and Invisibles can become untouchable for brief periods of time, Large Bodies are invulnerable from the front, Hot Rods travel over the stage very quickly and deal high knockback, Green Requiems can heal fighters nearby and Heartless, Parasol Beauties can fire energy beams from their parasols and can float in the air, and Invisibles can curse fighters with a ark shadowy ring surrounding them that ignore shields and must be jumped or dodge rolled through with good timing.
Behemoth: seperate from the Heartless spawn is the Behemoth Heartless, it can slam down into the stage with a big jump, rain down dark orbs it casts from its horn, cast thundaga spellls all over and can be riden/stood atop of and attacked by fighters. It is a very large AT on par with Rathalos.