Ability to capture multiple audio sources. Eg. mic sounds separate from desktop sounds.
This will allow viewers to listen to streamer talk while playing their OWN music. Often I want to watch a stream but I really do not like their music tastes. OBS and other software already captures these sources separate (mic and desktop) so implementing this shouldn't be too hard and would provide a GREAT amount of flexibility for watching streams. Also, it has more benefits as well, you can have 2 volume bars in streams and the viewers could also hypothetically adjust the music volume themselves too!
Bang Plays Games commented
This would be amazing, but understand that many streamers might not have the bandwidth to support this. Essentially every audio track added increases the necessary bandwidth by 160~320 kbps. At 6 tracks (current maximum for OBS) that means your audio could be as much as 1920kbps alone. Add a high quality 720p video stream and you're looking at 4420kbps total bandwidth required. I personally have a 5000kbps upstream availability from my own ISP (Unfortunately this is the maximum I can get where I live). That would mean almost 95% of a half-duplex connection being used on my upload reducing my download speed to 3mbps, which is unusable for the games I stream. It's an awesome thing to consider for those who can do it, but not every streamer would be able to.