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.
I think a better solution would to be to have the option to change the font size for the chat. I partake in streaming and if you connect your OBS to twitch you can have the chat next to the screen that shows what viewers see on stream. Within the OBS chat setting, you can change your font size easily and I've found myself keeping yup and actually be able to read chat easier. If only the Twitch site had this option cause I wouldn't need to zoom in the website and increase the size of the font.
I was just about suggest they add the ability for streamers to have multiple audio tracks, and for viewers to then be able to mute / change the volume of those on their own, say for people that don't want to listen to the stream's music or game sound.
OBS already supports multi-track audio so in theory, it should be possible to make a second track available on Twitch.