I got a few ideas:
-
Let people make their own multi-communities, then publish them. The multis would get a separate category in search.
-
Let community moderators decide to join a list. For example:
- community@a and community@b exist.
- mod@a starts multi-community@a
- mod@b requests for community@b to join that multi
- mod@a accepts
- multi-community@a is now a shared feed of both communities.