Ideally, one music track to associate with each environment background.
Not the first time this has come up. It’s probably the absolute lowest of priorities imaginable, as it has no effect on gameplay or on spending, and because lots of people just mute the game anyway. Despite all this, I want to bring it up anyway.
Yes, it would cost money and wouldn’t directly bring any in, but if done right it could make the game experience considerably less bland.