It’s kind of funny that you are talking about doing an A&A D-Day. I went to Gen-Con in Milwaukee, WI in 1999 and Avalon Hill had a booth there where they were displaying their newest product, A&AE. I talked to an older guy at the booth that said that he had designed the A&AE game (I can’t remember his name for the life of me). Anyway, he said that he was planning on putting together an A&A game based on one battle and he specifically mentioned the D-Day invasion. I thought it was a great idea but obviously it never happened (probably because of what happened with Hasbro).
You may also want to include concrete fortifications where infantry get a better defense roll. You could also have machine gun infantry that have an attack value of 2 but require a second infantry to hold the ammo belt and reload (if the 2nd infantry is killed, the machine gun becomes inoperable and the remaining infantry attacks at a 1). An initial bombing run & offshore battery would be good (before the landings), but I would only give each unit an attack value of 1 for this. Due to cloud cover and other factors the Allies barely did any damage. An excellent resource for this would be “D-Day June 6, 1944: The Climatic Battle of World War II” by Stephen E. Ambrose. It contains information on troop postions prior to and during the battle, where the airborne gliders and parachutes landed, and the units that were in every wave in the landings as well as a lot of other great info. Sea mines, land mines & barbed wire could also be added and would have to be cleared by engineer units. Perhaps morale can be used as well. When the US troops hit Omaha, they didn’t all rush forward headlong. Some huddled behind a nearby cliff and were only pressed into action by Sergeants that gathered them together and went forward. Ost Battalions (made up of foreign prisoners) also made up a good portion of Germany’s defensive force in Normandy and would surrender a bit easier than the German soldiers would. The Germans should also have heavy artillery (attack at 3) and you may allow them to attack at 4 because they had every inch of those beaches mapped out to the exact for their artillery (this would balance out the constant offshore bombardment).