Going along with what others have already said, I want to see something bigger, more original, and more challenging, more akin to Unleashed. A story with more meat to it and and big hub worlds to explore would be great, as well. Despite their critical reception, it seems like the recent games have just been smaller projects done on a tighter budget by the American studio (kind of like the Storybook games, but with less emphasis on the story). I wasn't able to appreciate them as much as most people, because it just felt like so much was "missing".
As far as other things I would like to see, a new Riders game (minus the unnecessary Kinect peripherals) would definitely grab my attention. I don't hate the All-Stars games, but they aren't really Sonic games, they're SEGA games, and they seem more modeled after Mario Kart. With Transformed, I feel like they focused specifically on the same audience they were aiming for with Colors and Generations, and that kind of pushed me away from being interested in it.
Another digital/mobile game with Sonic 4's graphical prowess but designed more like the recent handheld games (Colors DS, Generations 3DS, etc.) would also be really fun to see, especially with all of these Android-based game systems on the horizon. Now that we have physical controls and more powerful chips, I would like to see them push it even further.
