Call of Duty 4: Modern Warfare Prequel to come: Rumor
If details revealed by a trusted source is anything to go by then, the next Modern Warfare title will not be a direct successor of Modern Warfare 2, but rather a prequel of Call of Duty 4, and it will not be created on an all new game engine. This info has been leaked to thisisxbox.
The gameplay story will be mainly about the character "Ghost", which first appeared in Modern Warfare 2, and the gameplay events will revolve around the things which led to war in Call of Duty 4.
Neither Activision nor Infinity Ward have so far commented on the rumor, so its still wait and watch situation for all of us.