Audio Only Setting for Viewers
This would be pretty much the mirror opposite to watching a muted stream. In the stream resolution settings there would be an option to turn video output off and only listen to the audio (in high quality).
This would be really useful for people with low internet speed (also to reduce the use of bandwidth when a lot of people are accessing the service) or who want to do other resource demanding tasks that could otherwise suffer from having a high resolution stream run in the background.
Should be easy to add as Twitch already has audio-only option in the Android app (and youtube-dl confirms that there is in fact an audio-only stream available), however it's not available as an option in the Web UI.