A recent hands on preview of Sonic Generations posted in digital spy has some interesting info. It seems you may be able to go to Flying Battery Zone inside of Spagonia.
Confetti rains down from the skies like multi-colored rain, and Robotnik’s airship looms menacingly in the background. After racing across the quaint cityscape, classic Sonic scaled a Big Ben-esque tower and climbed up the clock hands.
A spin attack performed in the center of the clock face caused it to swing open and collide with Robotnik’s airship, enabling the hedgehog to climb on board. The outcome was entirely different when modern Sonic reached this juncture in his version of the stage.
Aided by the power of his boost metre, the contemporary character propelled himself to the highest point on tower and used it as a launch pad.
Then there’s the case of some of the latest screenshots. In fitting classic Sonic into the more modern levels, they seem to have been greatly inspired by the past. Doesn’t this screenshot above remind you of Hydrocity Zone? Plus, remember the surprise of Metal Sonic’s level being from Sonic CD? Even though there doesn’t appear to be a full Sonic 3 level to be coming in the game, it does seem like bits of Sonic 3 may be found throughout. Who wants to bet that classic Crisis City will play an awful lot like Lava Reef?