I know very little about Sonic Generations. I'm avoiding trailers and gameplay to not spoil too much of the game, you see.
I do know that it will contain remade levels from the Classic, Dreamcast, and Modern eras of Sonic games, and I do know that the already revealed Green Hill Zone remake looks and sounds hundreds of times better than the one they tried in Sonic Adventure 2.
So here are five stages that I would love to see remade for Sonic Generations. Three from the Classic era, and two from the Dreamcast era. I don't really feel like the Modern games need remade stages and I can't really think of a stage I would really be excited to see remade.
5. Speed Highway- Sonic Adventure
Speed Highway was the stage in Sonic Adventure that I think came closest to classic Sonic's gameplay. But for a level called "Speed Highway", I don't really think it was fast enough. My ideal remake of this would be to keep the first section in 3D, and add a Quick Step section for the building run. Then let chubby Sonic play an extended version of the 'At Dawn' section entirely in 2D.
4. Green Forest- Sonic Adventure 2
I didn't like Sonic Adventure 2 nearly as much as the first, mostly because there was too little Sonic/Shadow and too much everyone else. This stage was one I replayed quite often. It's extremely heavy on speed sections, but there are lots of "how do I get there?" spots that made me keep wanting to replay it.
3. Spring Yard Zone- Sonic The Hedgehog
Spring Yard Zone isn't very difficult and is a generally slow paced zone, but I always enjoyed playing it. Rolling through the half-pipes and trying to get huge jump distances by building up speed with ramps is still fun, and I'd like to see what a 3D version of it could look like.
2. Flying Battery Zone- Sonic and Knuckles
This was always one of my favorite stages in Sonic and Knuckles. There's a lot of edge-of-your-seat moments with bottomless pits, a crazy amount of alternate paths, and a mini boss that basically kicked its own ass. What more could you want?
1. Chemical Plant Zone- Sonic The Hedgehog 2
This stage shows off all the best parts about classic Sonic games. Insane speed, multiple branching paths, and speed boosters that actually are put there for good reason. It's great level design, and I would like to see it remade...partly to just Sonic Boost over that stupid purple water. I drowned in there so many times.
I don't know how this game works or if they're even planning to remake somewhat obscure stages, but I would hope that Generations covers a wide range of Sonic's history...and stays closer to the classic side because that is what I really want to see.