I know it's a bit boring...
But they could do Charizard + Greninja + Decidueye if they want to save space on the character select screen lol.
Also someone mentioned in another thread that Pokemon Trainer could be Bulbasaur + Wartortle + Charizard. Which made me think that they could have Pokemon Trainer and Pokemon Trainer 2 (sort of a semi-clone of Pokemon Trainer).
Pokemon Trainer - Ivysaur + Charizard-Y + Squirtle
Pokemon Trainer 2 - Bulbasaur + Charizard-X + Wartortle
and then have regular Charizard as a stand-alone character.
Another option is they could have a famous trainer from the Pokemon games. E.g. Bruno with Hitmonchan + Hitmonlee + Machamp or Hitmontop
In response to the post above, I doubt they'd have Jessie and James because they are from the anime (although they did sort of appear in Pokemon Yellow and might appear in the new Let's Go games, so who knows, and also we know they considered Meowth instead of Jigglypuff for the original games, so they could finally make him playable).