Additional Tabs Inside Each Category
When you click into a game, you get 3 possible tabs.
"Live Channels", "Videos", and "Clips".
Now, I know that tags exist already for this sort of thing but if we're being real.. nobody uses them. Or at least a small amount of people do.
You could expand on this idea and add categorized tabs inside the game you wish to watch.
Take Speedrunning, Modded, First Playthrough, Achievement Hunting, very common types of content you find for example.
This will allow the viewer to separate the content they want from the content they don't. And at the same time for the streamers, it puts them in a segment of their own where they are only battling other streamers doing the same thing for views. No longer is a first playthrough streamer fighting for viewers with a 200 viewer speedrunner who owns the category.
You could even take this a step further and give the partners their own tab too. Not only then does the smaller guys not have to fight the partners for views, but it helps those smaller streamers find the community that prefer a chat with less than 50 people in them. And for the viewers that do enjoy a party of 500, it's right there on the Partner tab for them to find too.
As someone who spends a lot of time on Twitch as both a viewer and a streamer. This feels like it would be perfect. And I think every smaller streamer would also appreciate being found easier because the competition is placed away from you.
Bonus: If there was a "Modded" tab, there's actually a good chance that many viewers of whatever game it is never knew mods existed for it. They'll then get curious and check. So you're taking them to a new place to explore thus keeping them on the site longer. Or achievement hunters might start watching each other in specific games to spur each other on and help each other get it. You'll be creating actual communities that live inside these sub categories.
When I say nobody uses tags, I mean for searching. Not for in their own stream.
You'll find that people learn you're doing a first playthrough via the streamer, the chat or the title 10000x more often than they will with the tags