It would be neat if communities from different instances could federate with each other like the instances themselves do.
In my mind it would work like this:
If two communities have the same topic, and have compatible rules, then they could federate with each other. This would show the posts from both in a combined view in whichever instance you were logged in on.
For moderators there would be two types of posts.
Posts originating on their instance they would have full mod control over, and any actions taken on the posts would change the post for all other communities they are federated with.
Posts originating from federated communities I think mods should be able to hide in the local communities as well as a subset of other mod abilities, like sticky. However these would only affect the local copy, not the original or the other federated communities.
As far as I know neither Kbin nor Lemmy has anything like that, but I think it would be a great feature if either could make something like that work.