And that's exactly what happened on the Sonic Retro forums, as from was pulled yesterday due to a claim from Sega of America. Regardless, the list is still making rounds around the Internet. Interestingly, this seems to hint at DLC right off the bat for a fan favorite level, the Casino Night Zone.
From Game Informer:
Green Hill Zone
Chemical Plant Zone
Sky Sanctuary Zone
Speed Highway
City Escape
Seaside Hill
Crisis City
Rooftop Run
Planet Wisp
DLC
Casino Night Zone
Boss Fights
Metal Sonic
Shadow
Silver
"Death Egg"
Perfect Chaos
Egg Dragoon
Boss LastBoss(TimeEater) (That's just what the file calls it.)
But really, who wouldn't be able to guess that Chemical Plant and Casino Night would be part of Sonic Generations?