I could go for this time period. So many more wars to fight. This was the beginning of the American "Imperial" age via Manifest Destiny.

War of 1812
Mexican American War
American Civil War

The American Civil War was the of last war featuring lines of men firing volleys of musket fire into each other. It was also the birth of modern warfare, seeing the development of repeating weapons, the machine gun and trench warfare. After this, I think it might be difficult to play out battles using the engine, but I think the following 2 wars would also be interesting to see anyway.

Spanish American War
Phillippine Insurrection

Speaking of the Phillippines, that would have been a good place to include in the main game. The Spanish had taken the islands as a colony after Magellan's visit to the islands, and were always facing revolts up to their ouster from the islands at the conclusion of the Spanish American War in 1898. (Though at one time the British drove them out of Manila for several years) Not willing to grant Phillippine inderpendence at the conclusion of the war, the insurrection began against shortly after and lasted into the early parts of the 20th century.