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.
You are literally describing reddit. Allowing mods to federate communities together would be novel.
The beauty of the fediverse is that when one volunteer-run server goes down (as happens all the time) there is little disruption if your feed is filling with other instance’s content. You can’t count on these volunteer-run servers to have 99.9% uptime like reddit, they can disappear over night.
Same idea for communities. If lemmy.world disappears tomorrow there are dozens of communities that disappear with it; fragmented across the fediverse. If mods of those communities were federated with complementary communities on other instances then there is no disruption.
I don’t think that communities should automatically federate, it should be agreed to by the mods. But with the current population we can’t afford to keep identical communities isolated. Many will die a slow death when together it could have been thriving.