When I think of a future with Open Social Media, I imagine three pillars.
1) Microblogging (ActivityPub), for sharing thoughts with strangers, ephemeral community building, and broad+global reach.
2) Forum / Stable Community (no good solution yet, AFAIK), for discussing, planning, buying/selling locally. All within a distinct group of people, membership of which can be managed.
3) Peer2Peer (Scuttlebutt?), for talking directly to friends, family, nearby people, and easily sharing files.
Flarum is an open source forum system similar to Discourse but not tied to a Patreon account. It's still working toward a first stable release, but looks to be an interesting offer in the forum category. Will provide public/private group abilities, among other features.
@wion looks nice, thanks for the tip!
It's not exactly what I'm looking for, though. I think the real value lies in a community site, that works as a forum, more than a forum that works as a community site. And the most important part would be whether identity and access is federated/decentralized/distributed.