A big general problem I have with the fediverse (as do others I’ve spoken to) is that it affords not real capacity to foster both private and public spaces with good and convenient means of moving and connecting with people between them. From the lack of truly private DMs, to no private group chats or local only spaces … the whole idea of private spaces for when people want them seems to be absent from the fediverse creators and it’s a significant gap IMO.
In the case of lemmy, I can imagine private communities being rather useful and pleasant. That is, communities visible only by people who are members or who have subscribed, with membership being optionally open or closed to being invite only or requiring approval or something similar. Having both federated and local-only versions of these would also probably be nice.
The useful part would be that you could meet people/accounts in private spaces and then see the same person/account in public too, which should only foster community creation through personal connections and discovery.
Then, whenever people need a quieter and more private space for a particular conversation or topic, they can take discussion out from the public and shield it in private. While you might argue that this would stifle discussion (and I see your point), I think there’s a relatively natural equilibrium between our needs for public and noisy engagement and quiet/safe/private interactions. I think people would naturally move between these spaces as they need.