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.
require all participating communities to store ALL of the data.
Wait, what? No, not at all. There is no reason for them to redundantly store all the data.
Imagine the same concept but the data is just being aggregated. The purpose is that content gets more exposure and engagement not to create an archive.