I don't really understand why you people are suggesting that the next Sonic game will be in the Storybook saga. Sega keeps to a fair system of releasing a game that is canon to the franchise (or however you put it) first, and then releasing something from the Storybook spin-off series. (i.e. Sonic 06, followed by Secret Rings, then Unleashed followed by Black Knight)
I guess this would probably be because no one, at this point, can really predict what Sega would do next that would be considered canon to the franchise, so we all just go around suggesting what fairy tale or bedtime story world Sonic will venture into next.
Hmm, well Sonic's already used the sword gimmick, so that cancels out something samurai-related, unless Sega wants to try and fix what they did wrong in Black Knight, so frankly I'm at a loss for ideas, now. I'll check back later.