Multiple Audio Channels
It would be great to let streamers send multiple audio channels. For example splitting game audio, streamer voice, music, and notifications.
Often Twitch is open to the side while the viewer is doing something else, and being able to switch off or adjust the volume of certain audio channels so they interfere less would be very useful, and make it more likely for users to keep tabs open.
In addition to not having to rely on streamers to get their voice, game and music balance right. If one likes the music to be off and listen to his own, easy. And you can make it and "advanced" option so that people who don't want to bother with balancin the channels don't have to.