Also consider the idea of allowing usernames to be autogenerated from email addresses, particular CPF combinations, etc. A bit like how display names can currently be configured.
The use case is users who do not want people to have to pick a username, e.g. intranets. The problem is usernames have to be unique, so we'd have to do something to force uniqueness, like adding a suffix.

The cns_check_name_valid is coded to be able to alter $username by reference parameter if $return_errors is set to true. Currently it's just not something we do, but we can do because cns_join_actual has been coded to respect this API.