There's could still be one more Sonic game in the vein of his Wii titles coming down Sega's pipeline.
Sega developed two Sonic titles for the Wii called Sonic and the Black Knight and Sonic and the Secret Rings that were a little different from his regular adventures. In general, the two games weren't received particularly well despite Sega's intent to try something new with its lightning-fast mascot. Like them or hate them, we may be getting a third.
According to a TSSZ News translation of a Sonic Paradise report from a public chat with a Sega Mexico representative (in Spanish), Black Knight and Secret Rings are but two of a three game series. The rep said the games were "part of a trilogy of Sonic titles with the characters involved in fantastic stories." When corrected about the number of games currently in existence, he said: "No, it'll be a trilogy, just you wait."
The rep then reportedly contacted TSSZ and backtracked, saying: "I want to tell you that the comments on the chat that I made yesterday were misinterpretations by the users and listeners from the website ... I've no access to this kind of information in any way." However, his comments were said to be direct citations. The original Sonic Paradise report, with chat log, has been removed, most likely at the rep's kind request.
If true, this rumor doesn't necessarily mean that Sega will release the third Sonic "storybook" game on the Wii. Sonic could bolt his way over to the Wii U to complete the trilogy. Meanwhile, the rep also vaguely hinted that a new Sonic Adventure game would be released on the 3DS. When asked about another game starring Sonic sidekick Tails, he said: "Something like a Sonic Adventure 3....DS? I think that would be a great idea."
No matter what, it'd be a safe bet to say that we're going to see more Sonic titles on every future console. It's just a matter of what they actually end up being. If the idea of a new Sonic game similar to Secret Rings or Black Knight excites or enrages you, just keep in mind it's a rumor until official confirmation.