I mean, outside of Green Hill Zone we have it basically confirmed there will be a Sonic 2, Sonic 3, Sonic & Knuckles, and Sonic CD stage. That's already a collection 5 returning stages there easily.
I do think the game may have 12-14 zones, so I am curious if certain games get more than one stage or if they're sticking to one zone per game. This is by diehard Sonic fans, so there is a chance some of the odder games get represented, with Knuckles Chaotix being the most obvious next-step as there's a few things in Sonic Mania that were obviously inspired by Knuckles Chaotix. It'd be kind of interesting to see a take on one of the zones from one of the GameGear Sonic games.
There's a lot of possibilities for returning zones, but there'd already be enough to pick from if they just stuck to the five games they have named. But I am curious if they may pull from more obscure games, and then of course the new stages have me quite excited after Studiopolis Zone, which looks great.
I'm basically sold on Sonic Mania as a game, all that remains for me is its scope and things like what zones/bosses are in the game and the like. Since we have returning stages, curious how that split will work, how many zones, what returning stages are there and how will they be remixed and adapted, etc. I might generate a wishlist, but since the release isn't that far off I'm curious how much we may learn about before release.