They need to re-explore the COD4 time-frame. Every location of WW2 has been played out like a bajillion times, and Treyarch decided to can the modern idea and head right back into the past again for some godforsaken reason...why not keep up with the modern warfare idea? It turned out to be perfect, if you ask me.
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.
I second a WW1 shooter, although sitting in a trench for 4 years, or until you get killed by artillery (whichever occurs first) might not make the best game.
A game set in late 1918 when fluidity had returned to the western front might be quite interesting though. Also, the game could include many of the lesser known fronts, which overall were not very static.
Call of Duty 6 :Battlefield
WW1 shooter less known fronts
Online- big stretch of land in WW1 20-25 sniper spots
Campaign mode - pick a country USA, Britain, France or Russia. Follow the main events of that country in the WW1