Improve Bitrate + HDR + 120FPS/Higher Resolution
Higher Bitrate: (Above 10.000, with video quality selector can be managed so if someone won't use that high download usage, he can tweak it), we still have a really low bitrate, and that force us try to tweak OBS to reach higher quality with lower CPU encoding speed. Still this is not a miracle, and 6000 bitrate is so limited (even if there is a trick to go beyond and reach 8000 bitrate).
HDR: OBS doesn't support because it's not worth it nowadays since Twitch is not supporting it, but other platforms are starting to give a hint to this technology. I think would be nice to have this option since Twitch is being used in so many Smart TV's in the present. So they could enjoy Twitch in a higher quality. Probably this option should need Twitch partnering with OBS or any other streaming program to make it possible in the best way.
Higher framerate/resolution: These days we're having better connections that allow us to go beyond the old limits, we could make this work since right now is really normal to see a gameplay in 120FPS and even the next gen consoles are supporting it.
Twitch is the best stream platform, but our technical options are the same since almost 10 years ago. We should aim to higher quality so there are no quality loss between our viewership and our content.
It's time to wake up and make some improvements :)

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Nemo64 commented
The problem with HDR is that it requires 10 bit color depth to look decent. (OBS does not even allow it with 8 bit even though technically it's possible)
h264 supports 10 bit but hardware support is not wide spread. NVidia GPUs for example have no support for 10 bit h264 for decode or encode.
YouTube supports HDR streaming using h265 but they reencode the stream to vp9 on the fly. 1. because they always do that for different resolutions and 2. to avoid the patent issues with that codec. This is not an option for twitch since they don't do reencode for normal people.
Direct vp9 support could be an option. But there is no hardware encoder support as far as i know. But the decoder support is great. This could be an option for all of those with dedicated streaming pc's that can handle heavy CPU encode.
The solution would probably be av1 support. Here we already have some great hardware encoders but decode support on smart tv's and older smartphones will be an issue (software decode is an option)
The other issue is that twitch does not do reencode server side. So the client will have to map HDR to SDR if needed.
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ShikenNuggets commented
Twitch is the only thing stopping me from switching my entire production process to HDR. Based on some vague guestimations I did a while ago I'd say about 10% of all people who regularly watch internet content own at least one HDR capable display (I imagine that number is higher for Twitch viewers since we tend to be more tech-savvy, but I don't have the data to back that up), and that number is only going to keep going up and up.
The bitrate and resolution limitations make Twitch VODs pretty much worthless to me nowadays. I local record at like 5x the bitrate that Twitch allows, so my YouTube uploads of my streams are consistently much higher quality (not to mention it has a *much better* player), so I just tell everyone to watch them on YouTube lol
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The_Mese commented
HDR support is honestly a must these days. If OBS (and other platforms) support it, there's no reason why not implemneting it.
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Polyh3dron commented
Now that OBS 28 officially supports HDR, I hope HDR support will be examined on Twitch.