One element that I don't recall seeing in boardgames but which has ever been critical to military success (especially in WW2) would be some sort of intelligence/counterintelligence mechanic. Can you think of a method to gameify codebreaking and spies to manipulate how much information your opponents have about your forces, and vice versa?

As regarding anti-air-gun pieces, it might be more favorable to think of them as radar systems instead of guns - AA Flak guns were usually mobile, but they would depend on radar and other means of detection in order to be effective.