Reimu Hakurei. I can confirm she's coming because I personally donated an invitation to her shrine... Alongside the souls of whoever gave her any competition.
Don't worry though, they're chilling with Yuyuko in Hakugyokurou at the moment. they're in good hands
But if we're being honest, I'm risking a guess for Isaac as the Banjo of the pass. He's got much of the same situation as Banjo, just owned by Nintendo-Old, got an objectively better but lesser-known sequel shortly after and a divisive sequel much later, still managed to be popular for a long time and well into Ultimate's era. The only difference is that Banjo got in, and Isaac was snowglobed, after which his support kinda petered off.
That being said, I don't think ATs matter any more. Min Min wasn't chosen because the mascot was an AT, she was added because Sakurai went to Yabuki, Yabuki said "Everyone's the protagonist!" and singled out Min Min specifically, and so Sakurai went with Min Min. In addition, Min Min was a Spirit. Spirits were meant to expand the crossover beyond the fighters, like Trophies, but they were both easier to make and served a gameplay purpose. Cut to now, and a Spirit is now a fighter as well. ATs were meant to be a way for characters to take part in the battle while not being playable, and are toggled off on certain stages for various reasons (Doesn't mesh with the mechanics, already a background character,
doesn't have a reflection, etc.). If a Spirit can break free, so can an Assist Trophy.
So why Isaac
now? CP7 was presumably to be revealed at E3 alongside Min Min, and E3 has historically brought the big guns. In recent memory, we got the big request Ridley, Japanese cultural icon Hero, and another long-standing request in Banjo. If we're announcing the fan-favorite request, it would most likely be at E3-Except, y'know, there is no E3 anymore. A fan-favorite heavy-hitter would be a great way to truly start off the pass while giving Min Min hype beforehand, but that went down the drain with E3 and Min Min had to start things off by herself.
But hey, the Smash fandom is habitually wrong, and I'm most likely among them. But oh well, as long as CP7's cool.