I think for 2.0 the flarum guys are looking at it. its a lot of work I think though.
minimum the following:
- Implement ActivityPub
- Implement post scope
- Implement UI for external posts
- Implement management for federation (hiding instances/moderation etc)
- Post Handling of instances @ username @ instancedomain and stuff like that
- Handling other platforms, peertube, pleroma etc
That's just what I can think of, bear in mind im not a dev, my experience with fedi is running an instance so I just think of what I would require and some might be part of others. Though I wouldn't use any federation for my forum.