I always loved the game “Fortress America” which is an invasion of the US in the early/mid 21st century with modern (and in some cases futuristic: hovertanks) equipment. I think it would be really cool to have a game like this set in WW2 times, like if things went much worse for the Allies in Europe and the Pacific and Germany and Japan actually tried to take America.
I read a novel once called “Clash of Eagles” about a German occupation of the US Northeast – New England, New York, New Jersey and most of Pennsylvania. The Germans were hoping for a Japanese invasion on the West Coast but it never happened because of a devastating naval defeat by the US fleet around Hawaii. One important note about this story was that Germany had NOT invaded Russia yet and was still “friends” somewhat with Stalin. However, Hitler was still planning on Barbarossa even as the American occupation was getting shaky due to uprisings.