I was thinking, for example, something that an ActivityPub 'Place' (i.e., "type":"Place") could also have an "outbox".

Or is the question more like, should we only be able to follow Actors?

The more I mull it's really following updates to any collection associated to objects. i.e. Follow a collection to get a new chapter from a book. Follow the replies collection on a Place.

The specific JSON-LD would probably have to say “Service” or “Application” or else half of the Fediverse would lose their minds.

But yeah, it would basically be a bot, or a relay, or a group actor to coordinate activities between all of the followers.

Wanna make one in Emissary /Atlas Maps?