Let's see...
Sakurai wanted to put Mach Rider in Melee, but he and the team couldn't figure out how to make it work. AT is the next best thing.
You can't put Banjo as an assistant trophy (AT), that's impossible.
Geno could be an AT, after all, characters like Waluigi and Shovel Knight are AT.
The Chorus Kids, are from a rhythm game, not easy to work with, if they don't work, they are AT.
Ken as an Echo Fighter sounds plausible since he the Luigi of Ryu.
Same thing with Shadow, though he is more like Daisy. Oh and let's not forget he was an AT before.
Isaac, maybe he's not as popular (I mean the games) as other Nintendo franchises, but he looks like an easy target for a character.
Incineroar, almost like Isaac, except Pokémon is a multimillionaire franchise. However maybe it's not the most popular Pokémon Sun & Moon character.
So... to sum up:
Chances of being playable
10%
20%
30% Mach Rider
40%
50% The Chorus Kids
60% Geno
70% Isaac, Incineroar
80% Shadow, Banjo
90% Ken
100%
What do ya think?