I personally think this game will star Big Boss. Why? Look at the logo on his sleeve:
That's the logo for Militaires Sans Frontieres, the organisation that Big Boss starts in Peace Walker. Personally, I hope this is a reboot anyway as I think both Big Boss and Solid Snake have finished their character arcs and anything further will likely just be spinning their wheels.
EDIT: I feel I need to clarify some things. At the time of creating this thread all of the stuff I was reading said that it appeared to be Solid Snake (including the Giantbomb page). This thread was an attempt to get out in front of that. Also I don't follow Kojima rumors as I find them extremely frustrating (remember when MGS 4 was almost certain to come to 360?)