R.O.B., Hammer Bro., Lubba, Pauline, Petey Piranha, E. Gadd, King Bob-omb, Spike, Chargin' Chuck, Boom Boom, Pom Pom, the Sprixie Princesses, Birdo, Diddy Kong (heck I wouldn't mind getting several DK characters, it's still part of the Mario universe after all - Dixie, Tiny, Funky, Cranky, Kremling, K. Rool, DK Jr.)
Any/all of these characters I'd gladly take over the metals and babies.
And on top of that, they could take another page from Smash and add retro racers. Foreman Spike, Wart, etc.
There are so many good characters Nintendo. Why the babies/metals?