WWII is a popular setting for FPS games simply because that there are so many stories to tell for as massive a war it was. Games have done the North Africa campaign, D-Day countless times, the war in the Pacific, the Soviet defense... There's just so much that can be done with WWII, and so many different ways to do it. You have massive air battles like the Battle of Britain, island-hopping missions like Gaudalcanal or Tarawa, tank battles like the Bulge or Kursk, and a combination (D-Day).
WWI has been done once or twice, I believe (one of the more interesting ones was Iron Fist, or something along those lines). Korea would be interesting, especially the aforementioned Chosin Basin or, better yet, the landing at Inchon or the UN defense in South Korea.
Mogadishu has been done before, and rather well IIRC (DF: Black Hawk Down). BF:V covered Vietnam pretty well IMHO, and Medal of Honor/Call of Duty have done extremely well with World War II.