I posted a long response on their discord, but will summarize here:
Every A&A expansion has focused on US battles, so at least one of the 4 ideas will be US focused. The main US battles not covered in Europe are Italy and North Africa, and of the two I believe North Africa will be chosen (Rommel vs Montgomery, Afrika Korps, El Alamein, Tobruk, US landings… it’s a great, and self contained campaign):
So #1 is Axis & Allies North Africa
The next major European battle/campaign would have to be Barbarossa, or Stalingrad. Either would be good, Barbarossa is large scale land/air combat, and Stalingrad would be a first (city battle, encircled army, air resupply). I’d wager that Barbarossa is the more likely of the two, but Stalingrad might also be an option. For now, let’s go with the former.
So #2 is Axis & Allies Barbarossa
That leads us to the Pacific. We already had Guadalcanal, a couple classic options would be Okinawa or Iwo Jima but both would honestly be quite similar to D-Day in mechanics. I think Midway is the more likely option, anything larger infringes on the Pacific 1940 game.
So #3 is Axis & Allies Midway
Now, we return to Stalingrad, and also examine other battles. France 1940? We can play out Dunkirk, the Battle of Britain, and SeaLion all in one game. I think it’s a tossup between the two!
#4 is Axis & Allies Stalingrad (or others!)