Once upon a time, Metal Gear Solid: Peace Walker was given the title befitting a proper sequel to the Metal Gear Solid franchise. Kojima Productions public relations lead Jiro Oishi has revealed that Metal Gear Solid: Peace Walker was originally Metal Gear Solid 5: Peace Walker.
During the teaser campaign for both Metal Gear Solid: Peace Walker and Metal Gear: Rising, the number “5” would be prominently displayed. Since then, Kojima Productions dropped the number from Peace Walker.
Despite the seeming demotion from the franchise series, Hideo Kojima himself has clarified that Metal Gear Solid: Peace Walker is still a fully-fleshed out sequel in spirit. Regardless of the numberless title, Peace Walkeris the next Metal Gear.