Take Kirby, for example. Some of the past few games in the series (Epic Yarn, Mass Attack, Squeak Squad) each had their own unique element(s) to them, each of which might be referred to by some as just gimmicks. What makes most, if not all Kirby games such high quality though is that none of these gimmicks really change the core gameplay of a Kirby game. When we play a Kirby game, we know just about what to expect, with just enough gimmick to keep it fresh. Epic Yarn took a slightly bigger risk by completely removing Kirby's suck/copy abilities, but it was executed very well and still felt very "Kirby."
Sonic Unleashed was a different story. The main gimmick, our good friend Sonic the Werehog, infiltrated the game by not only introducing a completely non-Sonic playing style, but by giving it a 4:1 werehog-to-hedgehog playing time ratio. The werehog stages were drawn out and essentially just make people savor the day stages that much more. But I have a theory: Sonic Unleashed was not necessarily a bad game. It was a bad Sonic game. IMO the combat felt refined, and the platforming in the night stages was well thought-out. But it was DRASTICALLY different than gameplay in any other Sonic game, so fans (like me) were disappointed to find that the entirety of the speedy Sonic-style platforming accounted for about 30 minutes of gameplay in the entire game.
Sonic Colors is a perfect example of a well-executed gimmick. By adding a new, vibrant feature to an already proven platforming style, the game felt right at home while staying fresh at the same time. It's for this reason that I feel like had Sonic 06 been given the development time it deserved and gotten proper Adventure-style physics implemented (as opposed to the entirely auto-scripted antiphysics featured in the final product), it could have been a great game. Silver's psychokinesis and Shadow's vehicle abilities would have been just enough to keep each episode fresh to play while maintaining the core gameplay.
I feel like if Sonic Team stuck to the Colors formula for main console games, only fine-tuning the basic platforming while adding/changing specific gameplay gimmicks rather than changing gameplay as a whole, there would be little chance for another "failed" Sonic game. We don't need revolutionary ideas; just ideas to keep the kinds of Sonic games we already love entertaining.