I think that a huge opportunity for BI to take a step forward with regards to realism, in either ARMA 3 or ARMA 4, would be to develop a system of momentum energy transfer from a moving projectile i.e. bullet to a soft target.
IRL the energy that bullets dump into their targets is really very large;
Muzzle energy for standard speed 5.56 is around 1700 Newton meters and you can almost double that for 7.62 to around 3400Nm.
This is a huge energy with regard to a humans standard weight. For the 5.56 it is the equivalent of having a 170kg weight attached to a meter long lever which is attached to the body at the site of impact and for the 7.62, similar except with a 340kg weight.
Very few humans alive can stay standing unsupported with such torques applied to them.
I think therefore this is an opportunity for BI to develop their realism further by introducing this force transfer into game, even in a reductivist system which sacrifices accuracy of calculation for speed of calculation.
One thing I find missing in ARMA is the "first hit advantage" i.e. in a firefight the guys who land the first hits would typically knock over their enemy, or at least totally skew their aim, or cause them to momentarily drop their weapons even if the rounds are caught by body armor (most high quality armor these days will defeat 7.62 even at close ranges).
I think this area is really something BI could use to take another step in front of other offerings within the milsim gaming sphere.