Why doesn't Rock Band allow you to create additional cities and venues? Nothing fancy, just the ability to set a city name and country, then create some venues. Venues would just let you select from the existing arenas and clubs1 (the 3D animation tool for user-created venues feels like more of an RB3 thing). It seems like an obvious idea for selling more content: allow for users to download cities from other users or Harmonix, but require them to have x downloaded songs to be able to use the city. Maybe the venue creation could have a recommended genre for what types of songs to choose from a user's collection, but not require specific songs.
Except in one case: if a label wanted to set up a "city" that contained historic venues a group played at on their rise to stardom and require you to buy various tracks to use them, that seems like a really cool way for labels to increase artists' sales in Rock Band (or Guitar Hero). It'd be like a low-cost version of Rock Band: Beatles for any group that cared to take the time/ money to get the venues created.
1. I am, of course, ignoring the legal issues that could arise from letting people create venues with names like "This place in my hometown sucks b@!!s", but it's my post and I'll do so if I want to.