I think there are less hurdles than you'd think. Having content from another community served up when the feed is requested for the local community is a server feature not a federation feature. Moderators are the hard part, but in version one you don't need their powers to be federated.
It's the kind of thing you kinda have to just start trying (in a fork, say), then work out the kinks before putting the functionality into Lemmy. However, there are a lot more pressing issues at the moment, so it's probably something better left for down the line.