My brother came over yesterday and we spent most of the time talking about capabilities, petnames, DIDs, and the elusive federated MUD.
I think I have all the final pieces I didn't understand now to be able to design the infrastructure of the ever-elusive federated MUD (and yes it will be "activitypub compatible" mostly, though you will likely use a different "inbox" for a game stream).
The biggest challenge will be finding the time to code it, and a decent UI.