Many of the fediverse fans here will be familiar with https://a.gup.pe/ which provides groups to fediverse users. You can think of groups kind of like directories of users or mailing lists. If the group owner sends a message to the list everyone gets that message. Users sub/unsub by simply following or unfollowing the activitypub username/group name. Others in the group can reply to the messages and those get distributed also.
You can also allow anyone in the group to send a message and start a discussion. If we enabled this in Discourse this would allow anyone to start a topic and replies to that topic would show up in the thread.
This could drive a lot of engagement from other platforms on discourse via the fediverse. I think extending the activitypub plugin to enable this group management would be great. Adding extra features such as banning a user from a group to fight spam would also help.
Ric