The one that will never happen - Super Mario RPG 2.
But out of realistic games:
Definitely a 2D Mario. Something fresh. Time to discontinue the "New" series. Fresh (tighter) controls, fresh art style, fresh story, fresh mechanics, fresh gameplay. NSMBUD was mediocre and only was a blast when playing multiplayer.
After that Switch still needs a Mario Strikers, one more great 3D Mario either Galaxy 3 or Odyssey 2 or something brand new, and one more Mario Party though I am not interested in MP there should obviously be one more. Also a Mario themed Ring Fit game!