This is something that could be very risky, given the racial issues behind the conflict, but would fit the mature storytelling of the Red Dead series and its theme of American identity.
For those who don't know, the American Civil War was a conflict between the USA and a rebellious state calling itself The Confederate States of America, led by their own President, Jefferson Davis, who wanted to retain their rights to keep human slaves - as the economy of the South was so dependent on them. It was a horribly bloody conflict, but one which forced both sides to be technologically innovative.
The US Army, for example, had a Balloon Corps which involved using hot air balloons to spy on CSA fortifications. The Confederacy experimented with early submarine technology.
Let's say it was part of the game. Could you see the PS4 or other consoles being able to handle huge pitched battles? When games like Skyrim attempted to simulate a war, they could only manage small skirmishes. But with the power of this generation, could we see something on a larger scale?
It would certainly add a unique element to the familiar Western trappings of the game, and would explore a chapter of American history not often tackled by video games.