The idea would be the servers would have shared ban/block lists and similar rules so that they can share the load of having open sign ups.
Basically a coop of instances to improve on-boarding. If you join the coop then you get added to the pool of instances that get assigned normies at random.
If the authentication was federated it’d be ideal as well but I assume this would be outside the scope of AP and would cause issues if you tried to post from your mastodon.social account from mastodon.world’s server for instance.
The authentication could be another service, split from Mastodon, Lemmy, Pixelfed, … that only gave that service. The instance asks the auth server about “user@instance: password” and the server just says “OK/fail”. That or sending the user to the auth server to get a session cookie.
