I'm just theorizing about what the new Mario 3D should be. It's probably already in development, but here's my concept:
A semi-open-world, like Lego City Undercover. It would be based in the Mushroom Kingdom and familiar places (like Delfino Islands and maybe microplanets), and it would be an open world where you can explore every nook and cranny as a seamless environment, instead of just being sent to different areas with a definite transition. There would be sidequests, and you could use the gamepad as a scanner (and to receive messages and information). It would have activities like using FLUDD and karts (and some street racing), and maybe a means of visiting outer space to explore the tiny planets of the Galaxy series.
Thoughts?