I like the idea, but it should be opt-in. So communities can decide if they want to "combine" with other communities. And I would go a step further and not force exact naming schemes.
But why? They could just agree on one and delete the other one. What ist the benefit of having topic@example and topic@otherexample when both show the same content?
It should be opt-out, the default should be content is federated.
And it is already opt-out, that is what defederation is.
Of course defederation is a blunt tool.
Defederation should be possible on a per-community basis.
Also, what most mods want from deferation is not "we don't want to be heard by others" but "we don't want outsiders to be allowed to speak here" which is different.
Although I just noticed that posts are not stored in their communities but as sequential numbers on their instance.
If that reflects the internal structure, then disallowing outside from posting in one community but not another is going to be a ton of extra programming.
What you propose and mean by "federated" (combining communities across instances by default) presents significant technical difficulties, because there is no central authority of existing communities across all Lemmy instances. Imagine someone sets up a new instance, and some user there creates a community "foo" and becomes its moderator.
Then this instance begins federating with another one, where a community "foo" created by a different user already exists. Which one is the "correct" one? Who is its moderator?