I have an idea. Perhaps add national wonders called Westward Assistance and Eastward Assistance, to represent the cold war. The Westward Assistance would allow you to build import versions of US units, such as the Patton, the F86 Sabre, and the Longtom Howitzer. Later on, you could build the Abrams, the F-15 Eagle, and various other modern US units. Eastward Assistance would give you T55s, Mig16s, and later on, T72s, Gvozdikas, and Mig29s. You can't have both Westward Assistance and Eastward Assistance, since the USA and USSR were rivals. Of course, some of the unitartstyles would need altered for it to make sense, but it's an idea.