"This isn't name, Smokey. There are rules."
Now that I've got that out of the way, I'd like to perhaps bring up the possibility that some video game companies are moving for elbow room in terms of subjects and themes. Certainly, developers need to think about what makes a profit, but I condider several games from the past year as indicators of people becoming more and more comfortable with events like the Vietnam War. It'd make for some very interesting gameplay. It's not archaic and overdone like World War II, but not as fresh in our memories and clean as modern warfare.
Vietnam was a counter-point in American military culture. Some might argue for the Korean War, but Vietnam is arguably the point from which war lost all its class and glory. A burgeoning media exposed the battlefield in ways that are commonplace in our times with stuff like Abu-Ghiraib and Chechens executing people, etc. I would endorse that they do this war, because they could provide a more mature look at the horrors of war than Treyarch.
But, you know, considering the trailer and everything, It's probably Modern Warfare, as in a direct sequel.