I disagree with the premise of this post. People who want the experience this post describes can get it already using Mastodon and various workalikes such as Pleroma.

Like a subreddit, the purpose of a Lemmy community is not just to label posts as related to a certain topic or distribute moderation duties, but to build a community with its own set of inside jokes, shared wisdom, history, norms, and rules. To give an example, pictures of swans are welcome on /c/pics or /c/birding, but at /c/geese swans are the enemy. Hiss!!!

Sometimes communities fork because groups within them have incompatible preferences. That's a feature, not a bug.

I do, however think community migration between servers, initiated by the moderators should be a thing. As anyone who's been on Mastodon for a while knows, servers shut down, or sometimes develop other issues that mean some people would prefer to be elsewhere.