I just thought that it would be great to have a FOSS app for interactive playback.
It would support this new multi track format (or maybe just zipped ogg) that would enable you to play a song and control the "mix" or volume of each track. By each track I mean drums, bass, keys & synths, voice, chorus, etc.
The cool thing would be that, when you play a song, it would have a preset mix and you would be able to modify it.
For example, you could eliminate the drums and add the alternate drum version the artist recorded for you. Also, you could up the volume of alternate arrangements... or eliminate the voice so you can sing.
The configuration file (zipped with the oggs) would contain the name of each track and, maybe, some icon for each track, so you could see, at the interface, the icons and names of each track and control them. Also, it would contain the detailed info of each track (bpm, length, bitrate, etc) so you could replace any of those if needed.
Time stretching capabilities would be incredible!
It would be the best app ever!