Well, lets assume there will not be another black ops 2, due to the vietnam war being past. If we fallow the COD timeline, IW and 3arc both never make a game before THEIR last game. By that I mean MW was before MW2, and MW3 is after MW3. I've never played any W@W, but from my understanding, each game moves farther on the timeline forward. Let's also assume 3arc only does games based in the past. That leaves two major wars between now and the current afghan war. The Gulf War and Iraq war. Lets explore each seperatly...
The iraq war almost cuts into the MW and MW2 timeline, and wouldnt be very different in theme.
The Gulf War, however, would be their only alternative. This is my prediction, and I believe the next IW game will be in space.