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.