Chat font size, color schemes, and potentially highlighting.
Selectable font sizes for chat that are separate from zooming the entire page with Ctrl+Mousewheel Up would be awesome, even if they don't evenly break. A really cool idea that I've had for a while is actively scaling text fonts, so things like "lol" fill the width of the chat frame, as opposed to longer messages being somewhat smaller.
Additionally to this, simple color-specific modes for different eye conditions - black on yellow, high contrast white on black, yellow on blue - would be immensely helpful.
A pie in the sky addition to this would be selectively highlighting chat messages from established friends or the broadcaster, or highlighting messages from VIP's for the broadcaster if they are the visually impaired one.