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.
OP didn’t say force. OP specifically said allow.
But then you have all the same federation politics and drama at the community level like we do at the instance level. It’d be such a mess.
Just make it easier for users to subscribe to each instance of a community with one tap. And let them view them all grouped in one feed.
And better yet, let people combine any communities together into a personal group; not just ones with the same name. There’s no need to get mods involved at all. I wrote more about this in another comment.