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/communityname@instance.name” on it so you know which community the comment was originally posted on.
I think there are less hurdles than you’d think. Having content from another community served up when the feed is requested for the local community is a server feature not a federation feature. Moderators are the hard part, but in version one you don’t need their powers to be federated.
It’s the kind of thing you kinda have to just start trying (in a fork, say), then work out the kinks before putting the functionality into Lemmy. However, there are a lot more pressing issues at the moment, so it’s probably something better left for down the line.