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.