Histogram on VODS based on chat frequency
Many times when watching VODs, you just want to see the highlights without watching the entire thing. This would make it easier to stay up to date on your favorite streamer or get a feel for a streamer you're interested in watching.
An elegant way to be able to do that could be with a histogram on the progress bar based on chat frequency.
A competitor recently implemented histograms on their progress bars based on most watched parts but given that the majority of watch time on Twitch is live and uninterrupted, having it based on chat would be more effective and unique to the live experience.
This could be expanded on down the line as well, by allowing you to see the frequency of specific emotes or words (for example PogChamp for exciting moments, or LUL for funny moments)

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Cauhu commented
Sometimes I'd like to only watch the best parts of a past broadcast. I think this could be achieved by showing "chat volume" or the amount of chat messages sent at every point in time in graph form.
Or maybe some other form of engagement would work, like a high frequency of LULs might indicate funny moments.
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mantis israel commented
the new highlight system would be more like soundcloud system where you have the comments in the timeline
you will be watching a twitch stream and you would have a "funny" button below the stream and maybe "highlight" button
then you press it in real time while watching the system will register all the people clicking those buttons in real time and at the end of the stream you could load the vod and see the timeline with all the highlights showing across the video the more people who clicked the "funny/highlight" button at a certain point of the stream it will show a higher graph curve on the timeline same as soundcloud audio visual display
then you could just skip in the vod according to the graph curve skipping to funny bits on the fly
or skipping to sad bits the graph could have multiple lines/layers each representing its own thing
you could even have an algorithm that just skips to the funny parts for you inside a vod
you would load a 6 hour stream of some guy playing GTA V RP and choose play funny bits and the vod would skip from one funny moment to the other according to the curve that was built by people watching real time
currently i just need to load those 6 hour vod streams and skip manually in hope i find the funny bits without missing them