There already is. World at War is set in 1942. Call of Duty 3 is set in 1944. Call of Duty 2 is set in WW2. Call of Duty Finest Hour is set in WW2. Read up on these games before you ask a question like this. It's obvious.
Call of Duty is done with WWII, they have been working on modern day for quite some time now. Treyarch has done World at War and Vietnam as Call of Duties. I know for a fact that IW is done with period games but Treyarch might not be finished with them.