There will be more subs over time dedicated to different topics that advertisers might want to target, one possible solution could be to simply add another sub filter.

Right now you get to toggle between 'recent' and 'top' posts, what if you could also hit a button to view 'boosted posts' for a given sub?

That way a dedicated sub wouldn't be required for ads, and the advertisers would get to be closer to the user-generated content without interfering with the main feed. As a result, ad rates will be higher than if there was a separate ad sub, and those ad rates would be sent right back to users as an airdrop.