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/[email protected]” on it so you know which community the comment was originally posted on.
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.
Wouldn’t a multilemmy still run into an issue where duplicate posts or cross posts show up multiple times in a feed?
Even if we wanted to solve that problem, right now there is no way to cross-post on Lemmy. There’s a cross-post button, but it actually does a repost. I think we should think about that when Lemmy implements a cross-post feature in the first place.