I have a few ideas:
>Japan, not Tokyo, but something with traditonal Japanese aesthetics
>Alpine valley, imagine starting at a glacier and things slowly getting warmer, greener and more populated as you go downhill, would work best in a 3D Adventure style game
>Open Pit Mine, Sonic's supposed to have themes of man vs. nature, so why not have a zone based on one of the most destructive things man can do to nature?
>Nuclear plant, same logic as above
>Marsh, Swamp or Estuary, we've seen the coastline as beautiful beaches and cliffs, why not show an uglier part of it
>Null Space (or something like it) as something you can actually play in instead of a QTE
>Hot Springs/geysers, technically in Chaos Island in Frontiers, but I'd like to see it in a Sonic Aesthetic.
>Something whimsical, like a Zone made of Candy. Dimps did it in the Advance games with Music Plant and Toy Kingdom, why can't Sonic Team?
>Zone made of meat or something equally disturbing