Discoverability through Ads
If you want people to run ads, give them a reason to WANT to run ads. The hardest thing to do on Twitch is get discovered. Allow people to be higher up, or at the top of, a directory for a certain amount of time whenever they run an ad on their stream. Cap it out so you can't do it all day.
People will run ads for discoverability AND they'll have a chance to grow.