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.
On a vaguely similar note, it might be cool if using the crosspost feature pooled upvotes from the various crossposts, and only let one of the crossposts show up in anyone’s All feed at a given time. It would make having multiple splintered communities for one topic less annoying, encourage cross-posting, and reduce spam when someone crossposts something to 5 communities and all 5 show up on your All page.
To really work I think it would have to pool comments together too - but then you run into issues with moderation. I’m not sure if there’s a good way to fix that issue.
Keeping communities separate is the simplest way to go, tbh. Sharing karma could lead to weird brigades, like r/ScreenshotsAreHard cross-posting from every picture of screens on the Fediverse and then mass-downvoting from there.
To me, the best solution would be to implement multireddits. That way, you can have your cat multilemmy of 100 communities without affecting your main feed, but you could also do the same for related or identical communities. Plus, moderators could create a multilemmy and display it prominently in their sidebar.
Being able to subscribe to a multi would solve that issue
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.