You list everything we want in the selections (maybe or maybe not open world), and I think the original Sonic Adventure describes the best possible 3D Sonic game, though in need of major improvements. I'd like a linear 3D game in the sense of the levels feature a path that is split, but you really only would run on a straight line. It gives the impression of 3D and keep the general feel of the original 2D games.
Or really, just make another 2D sonic game. The DS games keep up pretty well, I suppose. I'd like to see a 360 Arcade game with HD art, completely 2D, and just a flat out, honest-to-God 2D sonic game that doesn't look dated. What is most important is to forget Sonic '06 ever happened. Nothing that came out of that game was good at all. Plus, put it all back on Mobius. What ever happened to Mobius?