It seems as if with the latest trailer for The Phantom Pain, the veil is beginning to part. The game is indeed a part of the Metal Gear Solid series, as was widely speculated. It has been confirmed that it is the 5th game, along with Ground Zeroes. What’s unclear at this point, is if they’re two separate games or the same game, with different titles.
Considering this is Hideo Kojima we’re dealing with, it’s anyone’s guess as to how this will work. Generally, though, when dealing with Kojma, I just go with whatever the crazier option is.
While this is just speculation, I do believe that the main character is Big Boss in the trailer. From the 2:53 mark, you can see what appears to be a young Psycho Mantis and right after that scene, a shot of what appears to be Revolver Ocelot. Also, the person speaking towards the end sounds a lot like Robin Atkin Downes, who played as Master Miller, who was Big Boss’ second-in-command in the game, Peace Walker.