I understand that you refuse to limit the auto-complete to current team members in private repositories. What you should understand from the other issues I've seen on this is that the current auto-complete is practically useless.
- I am always much more likely to @ mention someone on the current team than someone off the team (for public repos, I'd assume I'm much more likely to mention someone who has already contributed, submitted a pull request, etc)
- I do not remember the usernames some of my team members have created (which are unrelated to their legal names)
- Given the vast number of user accounts, I can type my team members' full names and still not see my team members in the auto-complete list
Even if I only type "@A" - the top result should be one of my team members that has an "a" in their name or username - any partial match in my team always takes precedence over other users.