Here's something I've always wanted: A consumer music file format that preserves all of the underlying tracks ('stems') so that they can be adjusted while you listen to the song.
Imagine listening to a recording and going "ooh, that bassline", and then turning up the bassist and turning down everything else to hear it better. Or easily "remixing" while you listen by eg. applying a filter to only the drums. You could even save presets to listen to "your" mix of the song again. Maybe this only appeals to hardcore music fans, but I know it's something I'd love to play with.
Of course it would have to be supported by major user-friendly software like iTunes, and would have to implement a nice interface for interacting with a tracks. But I've always thought Apple was very well positioned to take this on, if they wanted to.