Hi there. I am moderator at SocialHub, the grassroots community where we work on the ActivityPub protocols and federated apps in general. I'd like to mention that Pavilion intends to add federation support to Discourse. They applied for a NLnet grant, which was rejected so their plans have been postponed.
If you are interested to go further with adding federation support you could check out the Guide for new AP implementers which has a ton of pointers. I am also co-maintaining three delightful lists, for apps, developer resources, and clients respectively. They can be found at:
I am active on the Fediverse myself (unsurprisingly) and often see people ('fedizens') proclaiming how they'd love to have a federated forum. Posting Discourse or Flarum links and asking people to participate on the forum poses a significant barrier as they have to sign up first with a new account (I have a ton of forum accounts and am hesitant to create more). What currently comes closest to a forum is Lemmy, but it is a link aggregator and lacks many forum features.
On Discourse forum I once created a brainstorm topic of what federation might bring, which may inspire you.