I really think the x-pack will deal with Africa and Asia's colonization. I don't think they'll put the American Civil War in.
Anyway, nuevointi, Latin American military history is not all that interesting you know? Sure there are a lot of characters, but their interest is in their political decisions much more than their military ones.
While I don't know much about South American history, I can speak for the Mexican one: from 1810 to 1923 we were on intermitent war with only three or four short periods were military tactics really mattered, everything else was decided by treaties or by other political artifices.
Notable periods or characters that could be depicted in this kind of game:
-Morelos (1813-1815)The true military leader of the independence.
-Santa Anna: War with Texas (1835)
-Mexican American War (1846-1848) This wasn't even a war, think of an invasion that couldn't be countered.
-Zaragoza's heroic defense of the city of Puebla, but that lasted only a day: May 5, 1862.
So, while the country was immersed in war, the really interesting things happened in paper. If you want to study Mexican history of the XIX century, please do, it's something really interesting, however, for military things, a designer would have to take huge creativity liberties to make work. (By this I'm not saying it wouldn't be nice, just a little bit difficult and inaccurate perhaps.=
Co-Author of the Aztec Civilization Outline for AoM.
Peter Jackson and Guillermo del Toro to do The Hobbit!
Chichén Itzá is one of the New 7 Wonders of the World!