Mario Motor Madness: A Simpsons: Hit and Run style game where at differnet points of the game, you control either Mario, Luigi, Wario, or Waluigi, completing missions around the Mushroom Kingdom and uncovering an even bigger plot. You'd drive vehicles such as those that are from the Mario Kart series and new ones. "Neighborhoods" would be the area around Peach's and Bowser's Castles, Yoshi's Island, and DK's Jungle. Why Wario & Waluigi and Mario & Luigi would be working together I don't know, maybe something would happen where the solution can only be if they work together. (it's a dumb idea I know, but it's one popular genre Mario hasn't gone into YET)
Man that was long.