The ability to migrate an existing email and password account to a login with SSO would be helpful. Personally I'm trying to keep logins to my google account for ease of access to services and the robust security google offer.
The ability to migrate an existing account to a different login type via SSO would be usefult to facilitate this feature. Maybe this could be added to the options after signing in normally.
The target would be to seemlessly migrate the account to the new login type transfering any and all data so the user doesn't notice a differrence apart from the change in login they requested.