In AARHE we plan to make the AA piece represent infrastructure defence.
This is much needed as the game do not model any of that (defences). Amphibious assault against German fortifications (eg. Western Europe) should be a very different story to a casual landing on one of the Pacific islands, or Africa.
Infrastructure defence would consist of coastal batteries, anti-air guns, etc.
As for balancing in this case, a modest study of historic specifications leading to a a good model would avoid any strange/funny situations.
