I think there’s a huge difference between Cold War era and post Cold War era. Sure, AoE2 has a huge timeframe, but the closer you are to the present the more is the difference noticeable. If you add the Soviet Union you have to make it a cold war themed concept rather than 1929-2019 concept. I guess you can make “Cold War” one of the eras, but then having USSR in the post Cold War era will always be weird. You could make it 1900 - 2023 and have 5 eras: Great War → Interwar → World War 2 → Cold War → Modern World. Or cut the interwar if you want 5.