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 asked the Lemmy devs about multi-reddits type subscribing and they said that it’s on their list but they need help developing it because they have a huge list. I like the multiereddits way because then the user decides and there isn’t extra mods (managers) sprinkled in.
That’s the way I think it should work.