Real Big Boss left a tape for his body double, saying how they both can live as being one dude, and that he would be totally cool with that. Body double listens to it, seems fine or even psyched up about it. He then goes to a Sony MSX tapedeck thing, flips the tape over, and on the other side it says operation intrude n313, which is the name of the mission that Solid Snake undertook in Metal Gear 1. Weird static plays, some people are saying that might be part of brainwashing the dude, who knows right now.
An obvious flash forward happens, body double seems pissed, punches mirror, weird shit, credits. Timeline says Solid Snake killed Big Boss's phantom during the events of MG1.
In the cassette tapes after this ending, real Big Boss comes off as a dude that gives no fucks about anyone in his life. Ocelot mentions his clone children, Big Boss acts disgusted like they're just jizz on his hand and that they aren't real people.
Zero has some of the best tapes in the game, his character was pretty much redeemed, which is pretty surprising. An old, charismatic, charming British man that wants to accomplish something grand but doesn't trust anyone around him. He still harbors goodwill towards Big Boss.