Probably been suggested before, but not going to look..
A 'classic' mode with it's own leaderboards would be nice. Where the only power-up is the 2x multiplier by default, so the game is more like regular Rock Band and the factor of luck regarding purple notes is removed.
This way you not only satisfy the people who want competition without the random factor, it will help practice for the full mode since the OD paths will always be active AND it will also help alleviate the boredom of grinding for coins. Make the coin rewards far less than the full mode, but enough to actually profit even on low tier songs.