I also think there's room for taking some inspiration from more modern cryptographic-key-based identity systems like blockchain identities and AtProtocol and Urbit - your primary identity as a user is an asymmetric cryptographic key, which you keep extremely secure; and then you can use this key to authenticate and encrypt messages over a public network or on a public data store, and you can derive sub-keys for use in various contexts. Instead of having to have different authorities keep track of usernames, what if every user in the system was identified with a public key from a global, cryptographically-large key-space?