Coming from a guy that shoots .40 primarily, NATO should stick with 9x19.
It's a sensible cartridge. Handguns don't win wars, and most of the world's militaries are most familiar with 9x19. In addition, it's light to carry and easy for beginners to shoot, while having decent power and great capacity.
Beretta PX4; 92FS; 92S; 8040G | Sig Sauer SP2022; P6 | Colt Python 4"; 1968 Detective Special 2"| Smith & Wesson M642 No-Lock | Ruger LCP; 10/22 | Daniel Defense M4V1 | Remington 870 | Alpine M1 Carbine | Steyr M1895 |1943 Izhevsk Mosin M91/30 | Walther PPS