A number of people have mentioned that FPS set it the American Civil War, the Revolutionary wars, boer wars etc would be fun.... ehhh, no. Not FPS';
'Right, now march in foramtion while firing, halt, spend 10 seconds relonding, fire again then use you're beyonet for the rest. That wouldn't be fun. Howver, RTS' set it that era, like Empire: Total War is going to be, would be very fun. And thats the truth.
As for the whole WW2 thing, we need another COD2. That game was truly epic.
Or maybe they could tap, like the Korean War or something. There were some pretty impressive last stand battles in that that would make for good FPS missions.