It's nothing but speculation at this point, but what we
do know is that
Activision Blizzard is planning for Call of Duty 5, as revealed in their lineup of future titles. Infinity Ward, responsible for the recent CoD 4, and Treyarch, who brought us CoD 3, have been taking turns on developing the series, and now is seems Treyarch is looking for
level builders for a "Call of Duty" title with stipulation that "if you're a fan of World War 2 shooters, then even better!" Either CoD5 returns to a WWII setting, or they are developing an expansion for one of the other titles, which is rather unlikely.