I feel like the ship has sailed on streaming platforms. Trying to take on Twitch/YouTube, even if you’re Valve, seems like a losing battle.
A lot of people are not happy with Twitch. The discoverability on Twitch is abysmal. The big channels are getting bigger, and the smaller channels get buried under sub categories and never have a chance. Recently they added a "you may also like these channels" but it's pretty bad.
Content creators are also running away from YouTube. Some channels are doing ok, IRL, but overall it doesn't seem like YouTube cares about improving the live streaming part of website. They're so behind Twitch on many things.
I feel like a lot of semi popular Streamers, not yet Twitch partners, would jump at the chance of getting front page exposure on a Steam streaming platform. It would basically be like the people that got in first on Twitch.
And Steam already does tagging, content curation groups. I feel like they could do well with discoverability if they tried.
You could point at Mixer... But Mixer is nowhere near Steam. Every pc gamer has Steam installed.