I would Love to see fully detailed vehicle interiors, for example the tank interiors in Steel Fury - Kharkov 1942. Also realistic ballistic & armor penetrations and damage model of vehicles. A feeling of weight to the heavy armor(vehicles seem real bouncy, like they don’t weigh tons).
With the overall current great looks and open landscape combat of ArmaII/ArmaIII and some of the above implementations added it could be the best military combat sim of all time!!!