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 thought that’s more or less how it’s supposed to work now: if someone on instance A subscribes to a community on instance B, the community gets cached on instance A; and users there can post to it locally (and see each other’s posts) even if it temporarily can’t re-sync with instance B.
Is that not how it works in practice?