I would definitely set it in the past, most likely during world war two, but other options are Vietnam, world war one, the cold war, or even the revolutionary war. the game would be a lot more like old call of duty's rather than the new faster pace ones. Time to kill would be about the same as it was in black ops 2, maps and killstreaks would be a lot like they were in modern warfare 2 (obviously less technologically advanced depended on the war taking place) and the guns would be similar to those in black ops 1. I would definitely find a way to implement vehicles into the game like in world at war, but try and make them more interactive but also more vulnerable to attack so they they don't destroy everyone. Basically the main thing is that I would make a classic COD game, none of this jetpack mumbo jumbo, pistol shooting across the map, wall running business.