@varx you mean implementation-wise, where my Watch action results in a list of subscriptions (recursively derived) listening for replies to each entry?
Apparently both #Friendica and #Misskey already have Watch feature, so stuff can be derived from there.
An extension might be and indicator about who is watching, just like Github issues have too (though invisible watchers are probably best, privacy-wise).
@humanetech I don't know what the implementation would look like, or whether ActivityPub would make it prohibitively difficult.
But the feature I *want* is "let me know of any new replies under X" with the ability to say "oh wait, except for the subthreads under node Y".