I've always wondered why assist trophies went from mostly generic enemies or obscure Nintendo franchises in Brawl, to full on popular requests in Ultimate. I mean, more than half of the assists in Ultimate could easily have their movesets written for them if they were given the chance. There's also the fact that trophies can now be damaged properly, get KO'd, and dodge/air dodge if need be.
Also, a good chunk of AT's have corresponding miis as well, meaning it's likely that the smash team feel a full inclusion as of Ultimate wasn't plausible with the game's bigger focus on being a special case.
Edit: Still have my fingers cross that the playable quad and the Black Knight from Shovel Knight: Treasure Trove get mii fighters someday. Or at least both Shovel and Black Knight. If anybody feels like a fitting mii out of most AT's, it's Shovel Knight.
If Crash was a base fighter, what do you guys think his franchise's assist trophy would be? Personally, I think it would be cool if they went obscure and made Carbon Crash the AT. Otherwise, easily Coco, Crunch, or one of the three animal buddies.