Some questions...
What abilities are Sonic going to have?
Are there going to be other playables?
Will there be other modes?
What is the story behind the game?
Will there be any fan made characters?
Will this support online play?
...and now for some suggestions...
Don't go overboard with the gimmicks (if there are any).
The bosses should not be to hard to beat (unless your losing advances you to the next level).
If there are voices, then hire some good voice actors.
Grant the beginners a Tutorial.