Provide a secondary audio/TTS channel for visually impaired viewers.
I use a TTS reader to read chat to my headphones, and mix it in with game audio and music. I've toyed with running two concurrent streams from my computer so that legally blind viewers could open both and have chat read to them in real-time, but it hasn't been stable enough. Not sure if possible, but if a secondary audio channel could be utilized by the broadcaster, those needing chat read to them could turn it on and control its volume to be able to keep up with chat.
Alternatively, an in-page TTS reader might also be a good solution. Again, not sure how feasible, but it would be lovely.