I always had an idea for SUPER MARIO UNIVERSE, in which it was 2D. It had worlds like the traditional Mario series, but each world was themed after a very specific Mario game. So World 2 might look like take on Super Mario Bros. 2, implementing the style into the modern age. World 4 might be like Super Mario World. One like Yoshi's Island and Mario Galaxy and Mario 64, etc. Each world would contain individualized formulas that have been tried and tested, but then there would always be mechanics to really subvert your expectations. So you would never truly know what to expect.
But in each world, the visual style would change to suit the Mario game it was impersonating, and the audio would have both new tracks and old homages to each of the older mario games. The last world would be a crazy convergence, where the Marioverse literally breaks down... each level would be an insane mixture of all the Mario games, and you could find yourself working through the gameplay quirks of five or six different Mario games all on a single level.