010101 I like that idea. Perhaps when you're browsing a discussion, a random ad disguised as post shows up. The user who made it might be called "AdBot" (no profile page, but the name could be configurable).
As for the discussion list, there could be ads that act like the discussion card, but they are a bit larger so the ad can fit. All ad posts will be labeled as "Advertisements" by a fake tag label - everything would be fake anyways, you'd be borrowing appearance of other elements, but there wouldn't be any functionality, eg.: there won't be an "Ads" tag that lets you view all ads, when clicking on the bot's profile or username, it doesn't take you to their profile page, because it doesn't exist, etc. - but the visual is there.
Or if someone has wild imagination, they could make AdBot a normal user account that posts ad discussions, which then randomly appear around forum. Since this will have to completely overwrite how certain Flarum elements work (randomly showing ad discussions through the ad list, the bot's profile page...) I'd say this would be a premium extension sold separately as addon for this one.
I think that this could be a smart and a fun way how to display advertisements. Maybe I took it way too far, but I'm a dreamer