should be fairly easy: send invite to same community on other instance, after merge instances will sync community, moderators will merge.

But this might bring other problems

I think multi reddits (which are a highly requested feature now) could be used as a decent compromise.

Basically imagine users can group communities together into one mini-feed. This could be used for similar communities across multiple instances, like [email protected] and [email protected]. Call them multi-lemmies or subscription groups or web rings or whatever.

Then, what if moderators could cooperate create their own recommended feeds that users can subscribe to? Maybe even put a link to it next to the “Subscribe” link in the sidebar. If users of both communities are encouraged to sign up for the multi-lemmy, then everybody can see everybody else’s content in one combined feed without having to cross-post.

Posts are still hosted on their home instance, so there’s no extra work for moderators except over agreeing which other communities can join the ring. If a user doesn’t want to see a particular community for whatever reason, they can still subscribe/unsubscribe to specific communities like before. That way we get the best of both federation and similar communities sharing content.

You subscribe to a instance of a community.

Then within that community interface should be a list of all external instances of the same name,
that you can just click all the check boxes to sub to those too,

You now have a feed of that instance name that pulls from all of the instances,

and you can opt out of individual instances being in your feed, of ones where moderation is particularly bad for that particular instance.