This isn't a specific idea for any characters or anything, just a concept that came to mind. What if both consoles had their own specific guest character, but both Sony's and Microsoft's character are COMPLETELY identical in terms of gameplay, moveset, hitbox, etc.
They could be different characters from different franchises that are similar enough where they could have the same moves and stuff for both systems. This way it would be legal to use them in official tournaments since neither version would be at a disadvantage to the other because even though the characters are different, they are actually the same model.
It would give each console a unique feature.
For example.
SONY gets Deathstroke (DC) while Microsoft gets Jago(Killer Instinct).
Not the best idea, but it is just an example. They could essentially be the same character just different skins.