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 .40 S&W | Beretta 92FS 9x19 Para | Rock Island Armory GI 1911 .45 ACP | Colt Python 4" .357 Magnum | Colt 1968 Det. Spec. .38 Spc. | Ruger LCP .380 ACP | Daniel Defense M4V1 5.56x45 | Alpine M1 Carbine .30 | Steyr M1895 8x56 | Ruger 10/22 .22LR |1943 Izhevsk Mosin-Nagant M91/30 7.62x54R | Remington 870 12GA