While the final game ended up featuring a sort of rainbow note highway in a musically reactive city, this version is a much sunnier, somewhat more literal "highway" style environment. Somewhere between these two, the team thought of having the music make the city spring up building by building, but that didn't make it.
At one point it wasn't even decided where the music would come from. A DLC strategy was important to the team, but it wasn't certain that the game would be a Rock Band game. When it was decided that it would be, the source of music became obvious. And then the team had to convert all the songs from five lanes per instrument to two -- which, Nordhaus explained, made the game way too easy in lower difficulties. Finally, against his better judgment, the team playtested a game with only one difficulty, and it worked.
Senior designer Brian Chan explained that the game lost the "track-locking" mechanic from Amplitude and other handheld Rock Bands, in favor of a system that allows players to build "levels" in individual tracks. This allowed people to play in a much more freeform manner instead of following a prescribed, single best scoring pattern.
Even the specifics of the controls were in question, with considerations like gestural controls for each instrument. Above is an idea for keyboard controls that used left, right, and a button press to simulate keys.