Evidence is pointing towards the the next Call of Duty being set in the near future (about 2030ish) and I feel that it's the best way to go for the franchise. We've all fought against Nazis in the 1940s and evil Russians in the Modern day and, to be fair, it's getting a bit stale.
I'm not just hating on Call of Duty for this, but many other series as well. With games like Operation Flashpoint, Battlefield 3, Medal of Honor and Rainbow Six Vegas 2 all seem to follow the formula of evil Russians, Chinese or Taliban invade the Middle East or the almighty land of the free and are shown to be overly evil. Normally with some kind of nuclear weapon being involved.
It's fun and all that... but it isn't really innovating. However a game set in the future doesn't have to be confined to a "realistic" story as it's entirely fictional and it is up to the writers to decide how they shape the Universe they're creating. Homefront protracted a story in which the United States gets invaded by a massive Korean army in the late 2020s. You could have a game where the United Kingdom takes over Europe and declares war on the United States and it would make sense within the game world.
Whenever someone mentions a "Future Warfare" style game they mostly always think of things like Star Wars, Halo and Mass Effect with Aliens, Laser Guns, Teleporters and Space Battles. However "Future Warfare" make me think of things like Blade Runner, Homefront or Deus Ex which are set in gritty near future. I think that is the way the next Call of Duty should head.