You are literally describing reddit. Allowing mods to federate communities together would be novel.

The beauty of the fediverse is that when one volunteer-run server goes down (as happens all the time) there is little disruption if your feed is filling with other instance's content. You can't count on these volunteer-run servers to have 99.9% uptime like reddit, they can disappear over night.

Same idea for communities. If lemmy.world disappears tomorrow there are dozens of communities that disappear with it; fragmented across the fediverse. If mods of those communities were federated with complementary communities on other instances then there is no disruption.

I don't think that communities should automatically federate, it should be agreed to by the mods. But with the current population we can't afford to keep identical communities isolated. Many will die a slow death when together it could have been thriving.