Live video tool-tips that give the viewer more context, that can be customized on the backend by the content creator and interacted with the by the user watching the video. Possible applications could include educational instruction videos or how-tos.
My example uses a cooking stream, that allows the creator to add tooltips with info that may have not been shown during the broadcast, something I feel happens a lot in those videos. It allows the content creator to be terse and still offer more insight to the end user.
It could be potentially very useful in live and post streaming events.