You have Mario, Luigi, Toad, Peach, and even the hero himself, Link.
That is 5 characters.
I think there is at least three more you could add.
Now hear me out, because I have some great ideas.
1: Samus.
She has platformer games, and she even has what you could call power-ups, plus, there is already Metroid backgrounds and enemies in the game.
2: Sonic
Sonic is a series of platforming, and I think he would fit right in there with the others.
3. Shantae
Okay, I know she isn't from Nintendo, but WayForward has published their games on Nintendo devices. She is in a platforming game, she has power-ups, and she is a very good fighter.
If you really wanted to go all out, some other ideas would be Kirby and Mega Man, but I imagine those would be quite a bit harder to program.
Please, if you see this, take it into consideration.

Thanks for reading!