MGS V: The Phantom Pain E3 2014 Tralier Released With Major Story Spoiler Related to Snake
It appears that Big Boss, or someone who looks exactly like Big Boss is working with Skullface, the main villain of Ground Zeroes. I'm not sure if this is a Big Boss's clone, the man himself or if Kojima is trying to trick us and this Boss isn't really the Boss.
The major hint related to storyline plot of Metal Gear Solid V: The Phantom Pain is, "BEWARE Spoiler AHEAD"
Big Boss or someone who looks exactly like Big Boss is working with Skull Face, the lead villain of Metal Gear Solid V: Ground Zeroes. We have no idea whether he is the clone or Snake himself. Only Kojima knows the answer.
Just in-case the below video is taken down, here is a back-up.