Well I have no insider knowledge, but I'll venture a guess - something related to WWII desert theater. Basically, I eliminated the current lines already done (WW1, Napoleonics, Bulge, and SOHK) and the ones that don't make sense (Alamo, D-Day, Market Garden, Rome, Egypt, and FOB) and came up with just a few options - Africa, Civil War, AWI, and Crusades. I'm guessing Africa, but a Valley Forge one might be a decent option!
Of course, it could be one of the new lines or not follow my logic at all!