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.
Gotta say I like merged communities better than just multireddits. The problem we’re trying to solve is that one community of 1000 people is more than 10x better than 10 communities with 100 people, because instead of a bunch of posts or comments with less than 5 upvotes you get true content curation.
Would have to be voluntary and maybe there could be two levels, one where mods can only mod what is “truly” posted to their instance, and another where any mod can moderate anything in the combined community.