Font Size / Border Sizing
Allow users on Twitch.tv to hover over the borders of portions of the page (chat, channel side bar on the left) and drag them in and out to adjust their size. I want to use Theater Mode in a browser so I can still see and participate in the chat, but I want to decrease font size and shrink the chat a bit so I can increase the size of the actual stream.
Adding this level of display control across all Twitch platforms will greatly increase user experience and autonomy, and will probably negate the future need for developing a different viewing format (i.e. some other specific view setting besides Theater and Full Screen).