Fans know a lot about Sonic Generations thanks to demo-version files, so the current news will hardly surprise anyone. At the Spanish website Vandal information appeared, confirming the return of classic (or, more specifically, Sonic CD ) Metal Sonic in Sonic Generations as a boss, but both Classic Sonic and Modern Sonic will have to fight him. Not long after this, SEGA has published on Twitter and on Sonic's Facebook a model render, which you can see above.
Also Chemical Plant Zone from Sonic the Hedgehog 2 has been officially confirmend, and Special Stage of 3DS version will be like the ones from Sonic the Hedgehog 2 and other games, where the road Sonic is running on resembles a tube.
Update: in Japanese magazine Famitsu screenshots of Chemical Plant Zone and Metal Sonic battle were published. Looks like it'll take place in Stardust Speedway . Also you can see 3DS version of Special Stage . Click "Read More" for them.