It’s not quite what you’re looking for, but an even less ambitious approach would be to create a mini Reddit-like site on a single Discourse instance. I’m far from a Reddit expert, but my understanding is that a subreddit has essentially the same functionality as a Discourse category - with category moderators and possibly one or more Discourse groups associated with the category.
The easiest approach I can think of would be to connect a Discourse site to a WordPress site with the WP Discourse plugin. That would allow users to create new communities (categories) from the WordPress site via the API. An option could be given to allow users to subscribe to a category. That would give category moderators the chance to make some money for their work, and also generate some revenue for hosting the forum. DiscourseConnect could be used to assign users to the appropriate groups based on their subscriptions.
With the single forum approach, the unifying idea could be the site’s guidelines/tos.
An obvious issue with this approach is that a single Discourse instance can’t support an infinite number of categories. I’m not sure that everything on the web needs to be infinitely scalable though. UI issues caused by having lots of categories could be dealt with by having all categories muted by default.