To satisfy the Sonic fans, they might as well include 4 racers: Silver, Rouge, Blaze and the Chaotix. Personally, I'd love to see those characters despite only being a fan of the Chaotix. It would be cool to have the entire Sonic cast represented in a racing game. After that, they should go half popular non-Sonic characters and half cult favorite characters.
The question is, will this upcoming game be a full on sequel, satisfying all the hype we've been building, or will it turn out to be a cheap Wii U exclusive version with a few new racers and tracks?