I think a Sonic Colors 2 would be a great decision, because I really do think that the concept lends itself to expansion and that the game needs to be multi-platform. Of course throw the Wii U owners a bone and add in some second screen shit, but make it a game that doesn't rely on control gimmicks.
Plus, I really think it's about time that we get a numbered sequel again. No, NOT Sonic Adventure 3. But rather a numbered sequel to a recent successful Sonic game. Colors was, in my opinion and in the opinion of many others, the first really successful Sonic game in a long long time. And it was the first game since the Adventure series that deserved a sequel. Sonic '06 2? Fuck no. Sonic Unleashed 2? Oh hell no (besides, the werehog curse was cured, how shit would it be if the curse returned?). Sonic Generations 2? As much as I love Generations, it does not lend itself to a sequel given the celebratory nature of the game and the fact that another full game of referencing past titles would be overkill.
That leaves us with Colors, which had fun and unique gameplay ideas that deserve a bit more expansion. I'd gladly play a game with new and returning Color powers. Plus, why not throw Tails into the mix for a sequel, by allowing players to choose either Sonic or Tails?
Anyway, it's a pipe dream, but it's not something I'd be against. Nor would I be sad if it never happened.