Well, they can start by removing things like Cat Peach, Tanuki Mario, and any baby that isn't Mario/Luigi/Peach. The Metals at least originated outside of Mario Kart to some degree. Metal Mario was in multiple games, and Metal Peach was in Mario Party 2, and every Smash game since Melee. So Pink Gold Peach is fine due to that alone. You can also add Gold Luigi if you want any other Metals. Besides that, Donkey Kong Jr., Diddy Kong, Dr. Mario, Gold Luigi can easily replace the ones I noted are cut. That's at least one more space. So perhaps a WarioWare character(Ashely or Mona both could work well, since one is more promoted, but the other has a canon vehicle). And if you don't need Gold Luigi, just have both.