I THINK I FIGURED OUT PHANTOM PAIN
IT'S SET AFTER METAL GEAR SOLID 2
Big Boss's body is getting repaired from being destroyed by Snake. This is the same time after they've used Solidus's body to repair him, but he's not done yet - he still has an arm to go. Big Boss is hooked up to a VR machine while in a comatose state that forces him to prepare for battle and rehabilitation while in it. Ground Zeroes takes place after the Phantom Pain trailer, because it's more advanced in the program. The REAL game is going to be in two parts - Big Boss, who you play through his VR in, and eventually escape the hospital once he's unhooked from it - and part two, in which you play as Solid Snake and Otacon in Philanthropy, travelling the world and destroying Metal Gears, which fits much better into the open world theory. It would also make sense to the advanced technology we see Big Boss using in the Ground Zeroes trailer... Because it's modern times, but the VR puts him in the situation shortly after Peace Walker. It also explains why he looks so fucking old in the trailer, with the grey hair even though it's supposedly right after Peace Walker. Maybe the burnt man, and the reverse XOC thing is a representation of Big Boss's burnt body and subconscious?
I'M NOT CRAZY I SWEAR