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.
I mostly agree with this, but I also think there should be some way of being able to collate the same 5 communities on 5 different instances under 1 view. I said this when I first came onto the Fediverse, but maybe having a tagging system for each instance would allow for both; users could look up instances with, say, a “news” tag and get every instance with that tag - and this way, the communities would still be separate and can develop differently from one another.
Just make it like multireddits on Reddit. It allows you to collate multiple communities into one feed.