Characters that may never be played!

[The Vision]

BUT I WANTED TO DO SQUIRREL GIRL
YOU STOLE THE FIRST IDEA I HAD WHEN I SAW THE TITLE DAMMIT GUU

The Doctor (I feel like we had one)
He arrives, tries to be the Doctor, realizes he can't have any directly aggressive Moves because he's the Doctor and that the TARDIS in its full glory is not allowed according to the Rules.

Ash
It could almost work but the fact is that Ash has no skills of his own (okay he does have superhuman strength given the fact that he just picks up 100 kilogram Pokemon but you know what) and his moveset is entirely Assists.

Thomas the Tank Engine
I actually had a dream about this. I got Thomas as my third character and by Omni it was glorious. I imagined it as my old Optimus Prime character, but without the humanoid form. As in Thomas the Tank Engine with a turret attached to his chimney. It was badass. Unfortunately, I think that would go off the rails pretty quickly.