TINFOIL HAT THEORY WARNING

Just before I get into the meat of this theory, I need to point out that the floaty guy with the flapper-arm backpack in trailer 3 is Mads Mikkelsen - he does the same "shush" gesture, and the water rising lines up with his arrival like in trailer 2. Also trailer 3 comes before trailer 1.

He is Osiris - Egyptian god of the dead (hence Death Stranding), able to command the dead. He controls the flooding (of the nile) and the growing of the crops (start of trailer 3). He is one of the 5 floating figures, and the other 4 are his siblings - Set, Isis, Nephthys and Horus the Elder. He is garbed (in trailer 2) in green and black as is traditional for Osiris. He wields a gun in place of a crook, and a flappy arm thing instead of a flail.

The other world (underwater land) is the Duat, or underworld. It is day when the real world is night. If you're wondering why it's day when he returns, he returned with the sun like an Akh. The proximity to the dead enables a crossover between the two worlds.

The baby is an Akh (or the component parts of Ba and Ka?), able to traverse the Duat (saving Sam from the crater explosion) - which presumably is also how it is able to power the flappy arm that can detect the dead moving in the Duat. The dead aren't evil, they are simply controlled by the gods. The reason it seems to be in Sam's stomach, is because the Ancient Egyptians thought the mouth led to the womb.

The symbolism of the right eye in trailers 2 and 3 is that of the Eye of Horus - which represents the sun.

At the start of trailer 3, Sam is crouching down by a mummy with a golden death mask, performing funeral spells allowing it to transition to the Duat and become an Akh.

The cuffs (health bars) are Ankhs (not to be confused with Akhs), symbols of life carried by the hoop by the gods. Presumably they transition into cuffs when placed on a mortal - like the binds of an endebted slave. In Egypt such slaves would be stripped of all of their posessions, just like Sam has been in trailer 1 after gaining the item.

Edit 1 - Not just me seeing the connection https://www.reddit.com/r/DeathStranding/comments/7iib4k/golden_funerary_masks_possible_ancient_egypt

After some thought I think Mads may be a trick villain, because Osiris is no more evil than the Grim Reaper. The real enemy would be his enemy - Apep/Apophis, the Egyptian god of chaos personified as a giant snake. He lives in the Duat and eats souls - born from Ra's umbilical cord. This may be being embodied by the strands - which would also give justification for Sam's link to the sun/Ra - as Ra is Apep's nemesis.

Edit 2 - 0:37 the box on Sam's suit reads M2047 R4. = Doat Ra?

Edit 3 - 3:58 Mads has an artificial womb thing too, as well as what appears to be a gold mask. He points, appearing to be warning Mr Orange Suit.

Edit 4 - The middle of the 5 floating people in trailer 3 appears to be a mummy.

Edit 5 - Re: https://www.reddit.com/r/DeathStranding/comments/7ipngl/what_does_this_mean/

It means that the magic baby in trailer 3 is Mr Orange Suit's Akh. Note the brown eyes, not blue like in trailer 2. Guillermo has Sam's Akh. Sam has Mr Orange Suit's Akh.

Edit 6 - I should really credit 0235 for starting this snowball. Before he even watched trailer 3 he noted something from Assassin's Creed: Origins - that Pisces was connected to its other half in the Duat by a cord, and saw a parallel to Death Stranding.