Thanks for the replies, guys. I agree that categories seems like the easier way to do this. But, these would be subforums that users could create a lot. There could be hundreds or more (the subforums are a key piece of the app, so will grow as user base grows).
I’ve been told that creating hundreds/thousands of categories will be a big problem, so that I should look for other solutions. Do you have other suggestions? I am willing to build a separate app that interacts with the API. The key thing I want from Discourse is the outstanding post/reply/tag functionality.