@box464 I think we have almost all the building blocks in place to implement such feature (the recent development of groups and topics can be relevant as well).
It would require to build custom UIs and develop ad-hoc features on top of such building blocks to make it happen.

The high-level picture that cross my mind would be something like this:

• Alice create a new custom role called "Bob space"
• Bob space defines the circles bob can reach and how they can interact with him
• Family circle: made of 12 members, users in this circle can read, like, boost, mention and reply to bob's activities
• Friend circle: made of 59 members., users in this circle can read and like bob's activities
• Caretaker: made of 1members, Alice, user in this circle can also edit and delete bob's activities
  This would be the default boundary in bob composer, he could then switch between federated / local / mention only - which they would all be subset of the bob-space boundary
• Thanks to our "shared profile" feature (not yet enabled on campground), Alice can create a new user called bob, and invite bob by email to register with that user.

At this point bob can presumably safely interact with a custom audience, but he would be still able to see the feed and follow any users in the fediverse in different ways (navigating to a family member profile, looking to their followers and go from there).
We already have the possibility to limit features based upon boundaries, so we should enforce such rules across all the platform (eg. If I search for a username or hashtags, open a thread or look at user followers they should return only contents / users based upon my boundaries )

That would be a crucial features for enhancing safety on social network for different categories and usecases (another one that comes in my mind is about introducing children/teenagers on social networks and the internet at scale in a safe environment)