I also posted this in another thread, but:
What do you guys think about the following 3 fundamentals for a good 3D Sonic game?
1. Make Sonic control like a car. I think Sega has had the wrong approach to controlling Sonic in 3D space all along. Sonic is as fast as a car, so why does it make sense to control him like Mario? He should have acceleration, steering, and breaking controls. This in turn encourages the game levels to be built more along the lines of, say, a Mario Kart course, but hopefully with some platforming here and there, and with less linearity.
2. Use the classic Sonic design. This is more important than you might think. The design of Sonic drives the tone of the entire surrounding game. Using classic Sonic limits the voice acting, the edgy 'tude, and the shitty friends. It also encourages an overall aesthetic more akin to the Genesis titles.
3. Don't let Sonic Team develop it. Otherwise there's no way the old 3D games won't influence this one. Instead the game should be developed by an experienced action or racing game studio. Platinum Games would be the dream choice.