Meta-communities in two flawors:

• user/UI-based, where the user can create an entry in his preferences, add a bunch of communities there and set which one will be its "facade". The main "con" here that every single user should do a lot of work by himself
• server-based, where you can create new meta-community, add your own community there, and send invites to all similar communities to join. Every community that joined could(should?) be removed from discovery, but be visible as "nested". Search by its name should lead to meta-community. Ownership of meta-community should belong to its members in term that not a single one could delete it or make any harm. Meta-community exists while there's at least one member in it.

Important part that one to be careful about combining feeds. Small cozy community can just dissolve in valley of posts from much larger one. The thing is controversial, on one hand it's really handy to see all the posts all at once, on the other hand - it can kill smaller communities, as their creators will lose the sense and desire to have them maintained. There are different solutions: