Look, I know that this is pushing CoD close to the Halo zone of FPSs, but going into outer space and having futuristic weaponry isn't too farfetched for the series. In previous Call of Duty games, players have already spent some time floating around in space, shooting zombies on the moon, and getting their hands on super-advanced tech. Sledgehammer could make bringing CoD to a far--but perceivably realistic--future a setting that could work.
So far, the series has taken us almost as far as the year 2030, which really isn't that far into the future. What would really be interesting would be to go into the distant future, perhaps 100 years, when the world would be almost unrecognizable. Maybe in the future, new tensions will rise between nations (anything to make it so that we won't have to fight Russians again). Advanced technology could give players more gadgets to play with, and soldiers won't be shooting bullets anymore. If Sledgehammer wants to differentiate its game from all of the others, a radically changed future would be a surefire setting to distinguish yet another CoD.