Knowing Kojima it's not going to be a completely metaphorical/metaphysical thing, so that rules out afterlife type stuff. Nothing he's done in the past suggests anything as ambiguous as all that. He coats his weird in the familiar. There's also the future tech handcuffs going on, which seems to suggest scifi. Of course, how much does this trailer really reflect the game? Is this trailer representative of "reality" as it exists in the game, or is it something unreal but of the game like, say, Snake's hallucinations in MGSV? There's really no way to even remotely guess at this thing, but it's fun to try!
Kojima has apparently given some information, which can be referenced easily here
Of particular note from the Kojima interview to me is this:
You talk about “mass stranding” when there are a lot of whales or dolphins stranded on the beach. It’s called “live stranding” when they’re alive, and “dead stranding” when they’re dead. The meaning of the title is that something came from “some world” and is stranded.
Here's the best impression of Kojima's crazy storytelling that I can do, based on the trailer and what he's said there. I think the beached sealife carries a strong environmentalist theme, and that the state of our environment will be at stake, and that it will be our fault somehow. I think we never see how the umbilical cord is connected because it's a total bait and switch. The scar on the man's stomach is already long healed, they were never connected to each other, he didn't give man-birth.
Now, you might immediately think supernatural, that the "some world" is the afterlife or purgatory because of the imagery, but as I said before I don't think that's concrete enough for Kojima. I think he's evoking Evangelion here more than anything, and that the story will be firmly science fiction, albeit fantastical and nonsensical science fiction that has religious symbolism in it because why not. Personally, I'm thinking aliens of some kind. First contact.
Let's assume what one of the posters above me said is true, that somebody found evidence that the dog tags have formulas about gravity in relation to black holes (my google fu is failing me, I can't find anything about this). This is Earth (or a human colony on another planet) in the far, far future, after interstellar travel has been invented perhaps. Scientists create a miniature black hole, hoping to use it as an energy source despite the dangers (environmentalist theme, bam). This is not QUITE as crazy as it sounds, I've heard actual scientists talk about this in highly theoretical terms.
A quick aside to talk about cetacean mass stranding. There are, from what I can tell, many theories, but one sticks out as something that would make for a good environmentalist story: the sound pressure waves from human sonar and offshore oil drillling is bursting their sinus cavities and preventing them from being able to dive and navigate.
I think our miniature black hole was like a "sonar" that stranded a "pod" of five alien beings (the floating figures in the sky), just like whales, that they use the radiation emitted from black holes to navigate space, or themselves emit this radiation, and our artificially created one destroyed their navigation and trapped them here.
I think these aliens, perhaps, feed off the iron emitted from stars and an immediate source of iron is in the blood of living creatures, hence the umbilical cords jammed into everything. For whatever reason, perhaps because of our humanoid shape, the aliens use humans as a means to reproduce rather than a food source (there's an episode of Star Trek: TNG where Geordi has something like this happen to him). The large hand-prints on the sand at the beginning are the infant's mother, the man's wife, after she has been turned, coming to say goodbye to her baby while she still has some vague awareness of self. The empty handcuff was attached to his wife, a desperate attempt to keep hold of her even though she already had a cord implanted in her and they knew how this would end. She was pregnant when he fell asleep, and when he awoke to find her gone and an infant present he knew what happened, and grieved for his wife. He knew the cord meant the child would soon be gone too, although he was surprised at how sudden it was, it didn't affect him for long. The hand-prints all over the man are from other people who were with him on the beach, also with cords in them, who desperately held onto him after they were first turned, before losing their sense of self. The man doesn't get turned as well because he was in an accident years ago that required extensive surgery (the scar), and now has synthetic organs that pump artificial blood, making him useless to the aliens, who ignore him, although he probably has no idea that's why he's been spared.