So Dan Dawkins of PSM3 magazine had a really good, and well researched, article where he speculated on Metal Gear Solid 5. I suggest you go find a copy as it is a real treat for any MGS fan. On any case I thought I'd just give some interesting points from his article.
- The game will be set in 1970's (1974-1980 to be more precise) Africa and will feature Big Boss.
- The game will follow events after Peace Walker.
- Frank Jaeger and Revolver Ocelot will be returning.
- The plot will focuses on the philosophical dispute between Big Boss and Major Zero which were central to the MGS4 plot.
- Angola, South Africa, Rhodesia (neé Zimbabwe) and Mozambique were all mentioned as possible locations for the game.
- Might feature a young Solid Snake.
- The game might be Open World (though the author clearly stats that that feature could also be a "Project Ogre" related which at this point does not seem to be related to MGS5)
- The game will be using the Fox Engine.
- The game will probably be a Next Gen title and would probably be a multi platform release including the PC.
Again, check out his article. He goes into much finer details about his MGS5 speculations.