It seems a Dutch writer has claimed he has uncovered what the game would be - Sonic Adventure 3, which would fit in with Sega registering six Sonic Adventure domains in the past few months.
Other evidence has popped up for a Sonic title in general. For instance, Happy Tree Friends writer Ken Pontac has added credit for a new Sonic game to his resume, joining Sonic Colors and Sonic Generations. Rap singer Hunnid-P, who worked on Sonic Adventure 2, has announced on YouTube he's working on new tunes for a Sonic game.
Stay tuned for Sega's announcement.