Ok, I'll throw my theory.. Kojima after MGS4 was done with Metal Gear, and after the PS3 downgrade and the problems carried in MGS4 he focused in having its own engine and making his next game, and open world game with a story centered in the famous taboos etc.. the famous Project Ogre. But then in some point, I don't know if it was because of the Rising fiasco and the outsourcing of this game to Platinum he decides that he can't leave to anyone his child saga, and then he transforms Project Ogre in Metal Gear Solid V, keeping a lot of new gameplay elements that he wanted to do (new motion capture techniques, open world, day/night cycles, the taboo themes..) and introducing the story after Peace Walker events. I would even say that ponytail-horned Snake was originally a new character of a new game.
And then, even after everything I've said, I would even go to the point that this game will be a reboot of the entire saga and its mythos, and then after MGSV we'll have more Metal Gears in this "new canon".
