Dollars to donuts would suggest the next game could be set in Vietnam.
If using the pacific theatre in CoD5 as a testing ground for jungle-esque terrain design was any indicator anyways.
Other than that, I mean you really have to consider the fact that there have only been so many wars in the last 100 years for them to draw source from. Even if CoD : Modern Warfare wasn't based on Iraq/Afghanistan/Kosovo/Chechnya directly, there are enough similarities in the setting for it to be plausible.
They could do another modern conflict based game, but you have to decide where they are going to draw a story for that from when everything has been covered?
Frankly, i'm really excited that they are returning to WW2; because I loved the movie Letters from Iwo Jima - and this is based on that conflict a great deal I believe.