I'm convinced a Call of Duty Civil War edition could work.

[Scoobes]

No to the Civil War, mainly due to weapons. I really don't think it'd work for a relatively fast paced shooter.

No to WWI because if you think about what it actually was like, trench warfare isn't much of an FPS game. I think an RPG like game could work because to show the psychological and emotional impact of trench warfare. In an FPS you'd jump out your trench, you might get to the enemy lines, but most of the time you'd just get slaughtered. It was all about the defenders in WWI, and in a game I get the feeling it'd get very repetitive (what with pretty much every offensive ending in death).

No to 7 day war... too controversial. If done right it could be good but I can't think of any dev that could handle the content.

This leaves us with WWII, Vietnam and Modern War... which is pretty much what games cover, unfortunately. Korean war could be done effectively. Also, covering WWII from the view of soldiers from other nations like the Algerians who joined the French army but were treated as second-class citizens and cannon fodder (watch Days of Glory). Or perhaps from the viewpoint of a Gurkha soldier (From Nepal, they fight in the British army).