What it'll definitely be is a product of the FOX Engine, especially as Kojima himself has been busy posting ultra, ultra, ultra photo-real screenshots onto his Twitter feed. One was of a meeting room at KojPro alongside a picture of the same room rendered using FOX. Followers were challenged to identify which of the images was virtual, an almost impossible task given that FOX and REAL LIFE looked exactly the same.
Nothing else has been confirmed, but as we recently speculated via some indepth analysis of the Metal Gear mythology, it'll almost certainly be set in Africa between 1974 and the late 1980s. It's an obvious, glaring gap in the MGS timeline, which is odd, and slightly galling, for a series with such a well detailed lore.
Kojima Productions has been keeping its cards pretty close to its chest when it comes to specific details on the next Metal Gear - but with Hideo Kojima confirmed to be attending E3, we're pretty confident MGS5 will, finally, be revealed in L.A. in some form.