Pop out video player
Add an option to pop out the video player into a seperate browser window (the ones that do not display bookmarks and tab bar) that only shows the player and nothing else of the website.
For users with single-monitor setups this helps a great deal when doing something else while watching a stream. You can reserve a small portion of the screen for the beam player or you can have the popped-out player stay atop all other windows and play a game yourself while watching a stream.
Twich has this feature when clicking the settings icon and clicking "popout player".
Building on another suggestion: If this would be implemented it would be natural to also be able to pop out the interactive controls - or make them part of the popped-out video player window.