I really love the idea of a giant city where you can drive karts to locations too, but if I had to choose one Overworld theme for a future Open World Mario title, that would be the sky. I guess it would be similar to an Open World Mario Galaxy (something I would love to play too) but this time, you would travel the sky in a small old propeller plane and go through clouds of different colors and textures, rainbow roads, star roads, beanstalks, shooting flying enemies on your way to the next level. There would be a sense of discovery each time you see an island appearing in the sky after you passed through a cloud. You could sometimes land on an open sandbox level, sometimes on a short linear athletic level floating in the sky, sometimes on a village where you can take a break and shop… This is basically Zelda Skyward Sword remade in a Mario style, and much more ambitious. Actually, it seems the next Lego Star Wars game will be like that (but in space of course), I'm curious to see how it works.