Federation should ideally work not only at a server level, but a community level.
If there’s three different /c/aww communities across different servers, their respective mods should be able to join into a meta-community.
There is the potential for a less moderated child community to end up polluting the meta community, so membership should probably be able to be revoked by majority mod vote.
But aggregating a meta-community across each sub community could allow for a much more seamless growth between servers and avoid the duplication that’s inevitable by putting the onus on the client-side.