Sonic Generations 2 was orginally going to be a much more different game then it was before the final cut...
Multiplayer would have been a mode in the game, but was ultimately scrapped due to an early release. Shadow, Knuckles, Tails, and Metal Sonic would have been playable along with Sonic in this mode, along with your own Mii.
Mephiles got Scrapped
Along with Shadow , Mephiles the Dark from Sonic 06 was scrapped as the Classic Boss for the Modern Era for unknown reasons, possibly because it would make a major plothole in the game, as the events of Sonic 06 was erased at the end of the game, and Mephiles was destroyed.
Shadow the Scrapped Hedgehog
Shadow was going to make a return in this game, helping Sonic work up his speed and skills with Homing Attacks, but the role was replaced by Tails.