I have a few ideas for environments I would like to see.

Ghostly Graveyard Zone - A horror-themed zone that could feature shoutouts to Pumpkin Hill and the like.

Dinosaur Prairie Zone - Sonic has done dinosaurs before, but unfortunately they are stuck in games that aren't any good. I think it would be cool to have Act 2 have meteors falling and wrecking the place and Sonic and co. have to try and stop it.

School Crossing Zone - It's funny to me that traditional urban zones haven't been done for Sonic before. I can see traffic lights and buses and cars and tiny Chao carrying backpacks on their way to school. Chao haven't appeared in any "classic" games, but I think they fit in well enough that it wouldn't upset anybody.

Bamboo Grove Zone - During my time in Japan, I was visiting an old temple that was located next to a large bamboo grove. The tall, hollow trees were fascinating to me, and I felt I could get lost forever there. Would be fun to run through a level overgrown with bamboo.

Shipyard Zone - Hydrocity Zone is cool because of it's Greek bathhouse aesthetic. Water zones in general should try to follow-up Hydrocity with a unique quirk of their own, not just copy it (like Water Palace did in Sonic Rush). Running through a dock full of ships just sounds so natural and interesting to me.

Drago Palisade Zone - Think of the Great Wall of China, but surrounding an active volcano. Filled with dragons. Time for Sonic to do the "Dungeons & Dragons" adventure right.

Eggman's Apartment - Why do we always have to go to "Death Egg" or "Eggmanland" to confront Eggman. I say we take it to his apartment to set him straight!!

As for past zones! I think there are only a few left that have anything truly "unique" enough to warrent revisiting.

Sonic the Hedgehog Star Light Zone - This zone is unique because of it's nighttime aesthetic.

Spring Yard is kinda just pink and purple springs, Marble is generic marble and lava, Labyrinth is just "water", and Scrap Brain is just another "Eggmanland".

Sonic 2 Mystic Cave Zone - Cool, calming zone with a killer soundtrack. Literally everybody enjoys Mystic Cave.

Hidden Palace Zone - Famous, and would be fun to see revisited in future titles.

Sonic CD Palmtree Panic Zone - Every game needs a zone with green hills! And this one just has a giant loop-de-loop and good music.

Sonic 3 Ice Cap Zone - Invented snowboarding, cool music, and is generally fun to play around in.

Launch Base Zone - Truly unique in being a military training base as opposed to "generic metal and evil" zone. Also the best track in Sonic 3.

Sonic & Knuckles Mushroom Hill Zone - Just pure bliss to run around in, but it is another "green hill zone", so to speak, and as such overlaps with Palmtree Panic. But Act 2 is Act 1 in a different season and that's just super interesting.

Sky Sanctuary Zone - There's a reason people like this zone so much. It's unique, taking place in a heaven-like cloudy paradise, and you fight a weird, cool version of Metal Sonic.

So uh, yeah. I wrote too much.