I'm surprised there aren't more games based on the Civil War. The only one off the top of my head is North And South. But this was the era in which America legitimately split, effectively becoming two countries. It's odd to me that American devs don't place more games in this timeline. A "brother vs. brother" campaign would be an interesting take.
I'd also like to see at least one game that takes place during the rise of communism in China. From the founding of the Communist Party of China to the rise of the Red Army and events like the Long March and the rise of the People's Liberation Army to Mao Zedong becoming China's leader in 1949 to the disastrous Great Leap Forward from 1958 to 1962 to more "recent" events like the Tiananmen Square protests of 1989, this has always been a period that fascinated me, and I'd love to see a digital representation.