I think we'll continue to see exponentially more assist trophy promotions as the series goes on, as more and more assist trophies seem to be "consolation" prizes for characters with significant popular or moveset potential that just couldn't make the cut for one reason or another. With Brawl, most of the assists were more abstract (Helirin, Lakitu, Tanks and Infantry, etc.) and mostly represented smaller Nintendo franchises. Smash for Wii U/3DS placed a greater emphasis on adding supporting characters from franchises already in the roster (Ashley, Dark Samus, Skull Kid, Midna, etc.). Ultimate doubled down on this further, with characters that did moderately well on the ballot but not Ridley/K. Rool-tier well like Isaac, Krystal, and Shovel Knight being added.
I'd honestly be really surprised if we didn't see at least a couple of assist trophies upgraded. Waluigi is the most obvious contender, but Alucard, Ashley, Issac, Bomberman, Zero, Skull Kid, Takamaru, Shadow, and Knuckles all seem like they could join the battle next time.
He's just getting X and Zero mixed up since both characters are being added to Super Smash Bros: Crescendo in 2026