Sonic 1 - Scrap Brain Zone sounds good.
Sonic 2 - Metropolis Zone.
Sonic 3 - Ice Cap Zone.
S&K - Death Egg Zone, definitely.
Sonic CD - An actual Stardust Speedway. Think about it - Act 1 could be the "Past", while Act 2 could be the "Present". Either that or the Wacky Workbench idea.
Sonic Adventure - Lost World
Sonic Adventure 2 - Cannon's Core with multi-character. Both versions of Sonic could be used, as well as both Tails. Knuckles, Rouge, and Shadow could be thrown in too.
Sonic Heroes - Hang Castle. I liked the haunted theme.
Shadow the Hedgehog - Any Black Comet level.
Sonic '06 - Uhhh... Dusty Desert. Yep, that was a decent one.
Sonic Unleashed - Eggmanland! And like the original Act, you could alternate between both Sonics.
Sonic Colors - Starlight Carnival.
Characters - Tails, Knuckles, Shadow, Silver, Blaze. Tails and Knuckles could be playable in Classic Acts, while Shadow, Silver, and Blaze could be played in Modern.
Boss Battles - Hmmm... I'd have to get back to that one.
Overall, I think they should've had much more DLC than just the pre-order bonus. For cripe's sake, the back of the box says "Downloadable Content".