Communities on different instances about the same topic should have the option to essentially federate so a post on one appears on all of them and opening any of them shows you the comments from all of them. This way when lemmy.world is down its not a big deal because posting to any news community federates to all of the communities instead of barely having people see your post. Federation could be decided by the community mods and the comments can have a little “/c/[email protected]” on it so you know which community the comment was originally posted on.
It would be a frontend thing. Track separate communities behind the scenes but show them together in the frontend if the community settings tell you to.
I guess the problem here is there is no central server. Different instances know about different communities. You could have an instance side setting to show all communities with the same name together. However, this messes up location based communities (!politics[email protected] is for New Zealand politics, and merging with [email protected] would be a bad idea). It would also mean the control is taken away from thw community itself. Doing it in that way would make moderation complicated.
I think having the ability for a community to opt to join with others is a better idea, though I admit I don’t know all the implementation details.