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 agree that my idea probably wouldn’t be great, for the reasons we both stated. While multicommunities are a good idea, I’m not sure they address the specific issue bothering me either, of crossposts spamming the All feed. OP’s idea might help with that a little - but honestly, I just think the ‘Hot’ algorithm needs some more fine tuning, and perhaps custom logic to avoid showing duplicates.