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.
Federation already solves the issue you have. If every user subscribed to every instance of /c/cats, then they would all see every post and could comment on each of them. There’s nothing gained by having another level of federation other than making it slightly easier to subscribe to all of them at once.
Personally, I’d rather see user-controlled “multireddits”, but better. You group together any number of communities and give the group a name. Then make it easy to publish the group as a link that others can view and import into their account.
All we really need is any easy way for people to subscribe to multiple instance of “cats” with one tap. (And to unsubscribe just as easily). I think the best way to do this is with user-driven, sharable community groups.
For example, I could make a group that includes “cats”, “kittens”, “jellybean toes”, “cat photos”, “cat bellies”, “chonkers”, and whatever else. They don’t even need to have the same name. Then I can share that somewhere. Mods could put popular groupings in sidebars. Fediverse websites could have whole lists of popular groupings.
Plus you could have an additional feature: Lemmy could let you view one of your groups as a feed, just like you currently can view “Subscribed”, “Local”, or “Everything”. Sometimes you just want to see cat photos and not be bothered by world news or politics.
I like this idea and it sounds easy to implement without changing underlying infrastructure.
It would be nice being able to publish communities I follow and check out other people feeds, not only similar communities’
I still don’t get what the point is of multiple instances with the same communities. Your proposal is a fix for a problem that doesnt need to exist IMO. Just have one thematic community be on one specific instance, and a community for a different topic on a different instance
Any instance could go down forever at any point. Should all cat photos ever posted to Lemmy be on one instance? Hell no.
Besides, my idea is about combining any communities into a group. They could be duplicates like you’re assuming, or just closely related, like cat communities. Or I could group together several different sports teams communities into one feed to let me just view those communities when I feel like it.