@pfefferle Glad to hear you’re thinking of adding category and tag actors. That will work with Discourse, NodeBB and Lemmy, as long as you follow FEP-1b12 (which is what we’ve followed to support “taxonomy” actors)
We actually now do! The PR was merged 4 days ago:
In Discourse you select the ActivityPub username of the Actor while creating it and there’s a uniqueness check at that point, so in the case of such a conflict you’d have to make the new Actor’s name slightly different.
The only thing that really matters is that the ActivityPub username of the actor is unique on a domain, allowing Webfinger to work properly. A prefix is just one way to achieve that. You could do what the Discourse plugin does, i.e. ensuring preferredUsername uniqueness at the time of Actor creation.
I am assuming separation of concerns here. In the Discourse plugin ActivityPub usernames are, in some circumstances, initially populated from Discourse usernames (or category or tag names), but in all other respects are independent from them.