Let me start this off by saying that I acknowledge that the chances of this theory being true are probably pretty slim. But, I haven't seen anyone else say this yet (not to say that no one else has, I haven't checked the entire internet). Thus, I thought I'd write this out as a blog on the very slim chance that any of this theory ends up being true.
Also, spoilers for the MGS games. The only reason I say this is because I know a very specific user here who hasn't played MGS4 yet, despite my telling him to for YEARS, but that's his problem. He'll beat it eventually.
My first two pieces of evidence for this come from my secondary theory that what is seen in the trailer is in the United Kingdom. Why, you ask? Well, there's a UK flag in the hospital room.
Second, the vehicle seen briefly that the one armed man and bandage face are in has the steering wheel on the right side of it.
Now, obviously a flag and a right steering wheel car are not definite signs that the game is in the UK. But, the various theories about the game have thus far placed the location somewhere in Latin America, or the Caribbean. Costa Rica is the leading candidate, due to the whale tail in the Moby Dick Studio logo looking similar to a piece of land in Costa Rica. BUT, people drive on the right side of the street in Costa Rica, meaning that the cars there are left side steering wheel cars.
Now, I of course realize that there are many other places in Latin America/Caribbean other than Costa Rica such as Cuba, where Ground Zeroes (at least partially) takes place. And there are many nations in the Caribbean that were once British colonies, and do in fact drive on the left side of the street. But based on what I know about the Caribbean, Cuba is the only place with decent hospitals, and they drive on the right hand side in Cuba.
I know what you're thinking, "Moosey, you've really lost it this time." No, sir, I haven't! Remember in the Ground Zeroes trailer when it said that, "From FOX, two phantoms were born"? Okay, my theory is that Ground Zeroes is the story of one of those phantoms (Big Boss), and Phantom Pain is the other phantom, Major Zero. Those who played MGS4 already know a fair amount about the split between those two, and we've seen a fair amount of Big Boss's post MGS3 adventures in Portable Ops and Peace Walker. But we haven't seen anything from Major Zero.
Oh, and the part of my theory that is pure speculation on my part is that I think the soldiers shooting up the hospital are working for Big Boss. Why? Why not? Like I said, speculation.
Of course, the counter arguments to this theory are many. Such as the dude looking a lot like the various Snakes in the series, Zero having his left arm at the end of MGS4 (though, so did Big Boss). But like I said, I think the chances of this being true are slim. I just wanted to throw another theory out there to see what others think of it. Either way, Kojima's got me again, and I am very excited to see more of this game in the coming months, and I hope they really go into the one-armed stuff, and make that a central mechanic.