I have a wierd thought.... what if there was a newcomer that's just a original "made-for-smash" character? Not from like Subspace or anything, but like a brand new character made specifically for the game...
Even then, what would they look like?
I'm not sure, the closet thing I could think of a character is when we the player destroy Master Hand/Core, and the darkness spilled out of it, the darkness resonated with the players essence which gave birth to it conscious. I guess this character would be either a black silhouette with a wispy dark aura, or the opposite a white silhouette with a golden aura.
You would play as them as they embark on a existential quest of self discovery by defeating Smash characters. At the end you'll get an ambiguous ending and your free to play as the character who you have complete freedom to customize, even more than the Miis. You can choose between a boy or a girl, the height, you can keep them as silhouette, or buy or unlock aesthetics to change there appearance to cartoon like, anime like, or realistic like. You can equip them with different weapons, and abilities to base their entire moveset on: bow and arrows, swords (of different types) daggers magic guns, hammers, magic, ect, you can even equip them with items if you want.
Lastly like Soul Calibur, you can give them the moveset of a character in the games like a Soul of Kirby would give them Kirby's body and moves, Soul of Fox would just give you his moveset while keeping your body. The Soul... or maybe... Spirit of Sheik would give you her moveset...
If that's what Spirits could be, I'd be happy.