Lemmy

I feel user defined multi communities will get lemmy pretty close to this. Multi level hierarchical feeds could then be achieved by allowing the user to include a pre-existing multi-community in the sources of another, which would just be a front end thing, nothing new on the backend.

Beyond that, users and communities can self organise. Users can share and recommend multi coms while communities can do the same for other communities they seem themselves connecting with. Again, more front end stuff that should be pretty straightforward for sharing and importing multi com definitions, which would all just be lists of communities.