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 - Colt 1968 Detective Special .38 Spc. - Ruger LCP .380 - Daniel Defense M4V1 Carbine 5.56 - Ruger 10/22 .22LR - Remington 870 Express Tactical 12GA