Mario, Luigi, Peach, Toad, Yoshi, Wario, Waluigi, Bowser, Mii, Goomba (hey, they somehow swing baseball bats), Koopa Troopa, Daisy, Donkey Kong, Diddy Kong, Bowser Jr., Petey Piranha, Hammer Bro., Boo, Shy Guy, Dry Bones, Monty Mole, Fly Guy, Paratroopa, Wiggler.
Those are all characters that have been in past Mario sports games (there may be more, but I only checked a few), so I think they could do a full 11 on 11 games.
I am sure Nintendo could make it a good game. As for who would develop it, that would be a good question.