The guy with the bandage in the trailer, couldn't it be Python from Portable Ops?
Just read stuff like this from the wiki article:
Python and Snake fought in Vietnam together, and after The Boss disappeared in the early '60s, Python was considered by Snake to be the one person he could count on in battle. On one occasion, Python lost a poker game to Snake but was unable to pay up. Sometime during a secret mission in Vietnam, Python received a severe injury which cost him the ability to regulate his body temperature, and as a result, Snake presumed him to be dead. Without intense medical care, Python would overheat to the point where his body would literally burn itself up.
His body was recovered by the U.S. government and eventually put through surgery in which he was fitted with a specially designed cooling suit filled with liquid nitrogen, complete with metal shunts installed in his skull to relieve himself of excess heat. Because of this, he was able to freeze objects that were close to him by flushing the liquid nitrogen inside of his suit out into the environment, though by doing so he would lose some of the cooling agent, making him heat up slightly. He was also a superb marksman, especially with assault rifles. Due to the nature of his suit, he was difficult to engage in hand-to-hand combat. His suit also made him highly resistant to firearms.
After recovering from his nearly fatal injury, he was trained by the CIA as an "Anti-Snake" soldier, the first of his kind. Python was genetically altered and trained to be Snake's exact match on the battlefield, so that in case Snake, now known as Big Boss, were to turn against the United States, the CIA would have a contingency plan to have him eliminated. At some point, he was placed into the FOX unit. Due to his brainwashing and modification, Python grew to an almost sick obsession with perfectly matching Big Boss. To keep his abilities in check, he was regularly sent on wetworks assassination missions, which led to him having nightmares about those he had killed, and thus further driving him to eliminate Snake.
EDIT: It looks like Python joining you in Portable Ops is seen as canon