Here's my theory about everything.
At the beginning, it's obviously Big Boss. The original Ground Zeroes trailer had Chico in it, and this trailer has him rescuing Master Miller and it's showing Mother Base getting destroyed, so this obviously puts not too far after the events of Peace Walker.
IMO every single motherfucking thing besides those scenes I described in the trailer is taking place inside Big Boss's coma, even though that might not make a ton of sense (I'll touch on that.)
No clue who the bandaged face guy is, but if it's in the coma, it's either someone BB met before he went into the coma or just a random thing. Fun note: Bandaged Face guy in the gameplay demo is voiced by Keifer Sutherland...
As of right now, I'm not ready to say the supernatural stuff is real. The Volgin-esque character on fire could just be a memory from Big Boss. Same goes with the fire unicorn - could represent the Boss and her horse. Two things that kinda wreck this possibility though:
1. The Psycho Mantis-esque character with red hair. Psycho Mantis was not even a thing this early on in the Metal Gear Solid timeline, so at first I assumed it was just The Sorrow, but Big Boss knows what The Sorrow looks like, so his mind wouldn't envision him like that. BUT, another thing is that maybe since he's in a coma for so long, Psycho Mantis did find Big Boss in a coma and infiltrate his coma? Dude is psychic after all.
2. Ocelot is here. That's not a big deal, but it's obviously an older-than-MGS3 Ocelot. So either they ran into each other after MGS3/Peace Walker, or that is in the real world. But I find that hard to believe because a fucking fire unicorn is chasing after them.
I think that MGS5 is split up into two chapters, just like MGS2 and 3. MGS2 had Tanker and Plant, MGS3 had Virtuous Mission and Snake Eater, MGS5 will have The Phantom Pain (the coma/lengthy tutorial) and Ground Zeroes.
The Ground Zeroes portion of the game, since it's confirmed to take place in the 80s, will be about Big Boss rebuilding Outer Heaven in Zanzibar and setting up the events for Metal Gear 1 and Metal Gear 2: Solid Snake.
And then there's the finale. When it shows Big Boss on his motorcycle with the "Diamond Dogs" patch. I really have no idea what Diamond Dogs is at all. It might be the new name for his band of mercenaries, but the fact that whole thing is accompanied with Big Boss growing a motherfucking demon horn out of his head leads me to believe it's still during his coma. So I dunno about that.
And "V has come to." Could mean Big Boss coming out of his coma. IDK why BB would be called V. Another theory I've heard is that it means yet ANOTHER clone of Big Boss.
And one last thing: in the beginning of the trailer, after Big Boss is saved and goes into a coma, Master Miller looks at the camera and says "What about him?" so there's another theory going around that you play as Big Boss in his coma for the tutorial, but once you're out of it, you play as another character for the main game. No idea who that would be, though.
Sorry for the lengthy post, I've been hyped as fuck on this game for quite some time.
edit: no idea who the burned face/lizard guy is, though some people say it might be Volgin after being burned.
04/16 - Ringworm
04/19 - Trog
04/22 - Ghost
04/24 - Iron Reagan
05/13 - SLAYER