Imo Nintendo's biggest misstep is not creating a huge roster of Mario characters.
Egadd, toadsworth, fawful, and really any interesting character design from any number of spinoffs and rpg's should have long since been spin-off staples. Mario Sports games I get, since you're working with special moves and stats. Mario kart and party should both have behemoth rosters from this point and its sort of silly that Nintendo is still playing with small rosters and "what generic bad guy should be in this game?" issues
Even in the platforming games, even if there isn't a consistent story at *least* let the cast have a few more characters every now and again. Why is there kamek, magikoopas (that look like him) and also kammy, a named magikoopa that replaced kamek and was since ignored? Like, how hard would it be to have a bunch of magikoopas and kammy in the same game?