+1 for numbers above head instead of names, would be more RP and less meta if it will be implemented. also it would not be a problem to report on someone if he has number, same thing if he had "name".
also numbers should change for everyone every session or every day, because even numbers are rememberable at some point.
to exclude that moment when at one day one person playing with number A and at other day other person also playing with number A. we can use hexadecimal numbers and 7 digits. we will get F*F*F*F*F*F*F it would be 170859375 combinations. so if 200 different people will play on server every day, it will be 854296 days until we should use same number for different person.
and for mobile messages we can still use old players' names.