I think it'd be pretty cool to have members of the M16 series in the game. It's a weird omission.
The reason I choose the M16A1 and the M16A4 rather than the M16A3 and the M16A4 is to have an excuse for the stats being a bit different. The A1 could have more recoil and less mag size in exchange for a bit more damage while the A4 could only be burst, low recoil, but also slightly lower damage. I think it'd be a fun way to not only add a burst weapon, but a full auto variant.
Plus the style points of the old M16A1 and skin potential, ya know.