Circles functionality might be useful on the fediverse (the only thing I liked on the late G+), don't you think?

You could create lists of people (eg. IRL friends / colleagues / shitpost buddies / etc) that you could address particular posts to (or even say 'post to all followers except colleagues').

In Mastodon terms, the dropdown that has Public / Followers-only / Direct options would additionally have each of your circles. Effectively this would work like a group DM, except without having to @ mention (and posts would appear in the feed, not notifications).

In ActivityPub pub terms instead of the 'public' flag or followers Collection being addressed, it could address it directly to each circle member's actor, or to a Collection that represents them. Still need to think through how the list of members could be private (they wouldn't be public, but may need to be knowable to their members so that replies can be addressed, or maybe we just go further and have a Group actor for each circle).