I think a variation on the multi community might be a good idea but implemented differently.
Community’s could opt into a meta community (somethings that are the same name arn’t the same thing).
Then on the user client side when you subscribe you can choose to subscribe to the meta community. When your feed is constructed it doesn’t include de-federated etc. when you post it is via your instance to the meta community and a shortcut is posted to all the meta community members, and replies go to your meta post in your instance.