Let's say that the Fighter's Pass also came with one assist trophy per pack. Who would you have picked as the five assist trophies? The only rules is that the character must not be someone already physically present during any stage or animation (spirits are fine though), and the assist trophy must originate from the same series as the pack's character.
Persona - Jack Frost
He'd appear on the stage with a hee-ho before blasting the surrounding area with Mabufu, coating it in ice. Contact with the spell may freeze opponents.
Dragon Quest - Erik
Attacks by wielding knives. If he hits an opponent with his opening attack, Cobra Strike, they begin to take damage over time. But if Erik hits them while they're poisoned, his attack instead becomes Victimiser, causing the damage caused by his slash to increase sixfold.
Banjo-Kazooie - Clanker
Clanker appears as a massive behemoth. Like Chain Chomp, he's chained to the stage, though, but he still patrols the air as he swims around. If he is facing an opponent, he'll bite at them to try to eat them. If he succeeds, he does rapid damage as he grinds them up before spitting them back out of his blowhole.
Fatal Fury - Michael Max
A sort of upgraded replacement of the Brawl Little Mac assist. Michael Max will run across the stage, even entering the background via the Z axis to dodge blows, and unleash flurries of punches. Before he disappears, he'll also use the Tornado Upper, which can trap opponents in a rapidly damaging windstorm.
Fire Emblem: Three Houses - Hubert
Hubert von Vestra appears from the Assist Trophy and snaps his fingers. Suddenly, the entire corp of Vestra Sorcery Engineers appears behind him using Warp magic. Hubert shouts "We shall
burn together!" and takes a bow. The mage corp then casts Resonant Lightning to call down massive lightning strikes all over the stage. Hubert gives one of his trademark laughs before snapping his fingers and warping away with his battalion.
tl;dr I'm very sad that Hubert is the only retainer absent from the Garreg Mach Monastery stage, dammit.