I think some of the Mastodon problems about usernames and so could be solved by a federated trusted login/user server apart from the messaging server itself. So, a user could create his login on a server and use it on other servers, somewhat like a Kerberos/AD system
@b I think the login and messaging should be different things and an instance could be able to verify the user credentials on other instances, maybe with an web-of-trust system where instances could say which one they trust