I always had this idea in my mind for a massive open world Mario game. The world would have a ton of iconic locations like Peach's Castle, Toad Town, Sarasaland, Luigi's Mansion, Yoshi's Island, Delfino, Donkey Kong's Island, etc. You would play as Mario but could collect other characters as you progressed through the game being Luigi, Peach, Toad, Yoshi, Wario, Donkey Kong, and Bowser. All of whom have unique attributes. Throw in every Mario character imaginable as NPCs throughout the world. Even guys like Daisy and Waluigi. Not sure about the villain though. Maybe someone new. You could have all sorts of power-ups as well.