I also enjoyed Sonic R, although there was a lot that could have been improved.
More than five courses (and courses with more than one ideal route) would be nice. Also, it would have been nice if they could have increased the speed of the game to something more Sonic-like.
However, I'm hoping for a 2D Sonic game. If handheld Sonic games go 3D, that'll be the end of new 2D Sonic games, as hell will freeze over before they release another 2D Sonic game for a console.