I don't even know what genre this game is, how it plays, or what ever... but day 1.
This stuff looks so fucking rad.
Expect some pretty extreme horror elements. Kojima never shied away from using pretty scary supernatural stuff/opponents and creepy undead enemies in his games (e.g. The Sorrow fight in MGS3 or the de facto zombies in MGS4), but now that he is unshackled from the whole Espionage/mostly grounded in reality setting he can go full out.
What I think having seen the trailer:
I think this has to either be a Evil Within style nightmare setting where everything happens in the characters minds, take place in alternative/mixed universes or timelines or involve some sort of time travel. BOW gone wrong or alien parasitic life form sounds good, but alone doesn't explain the following:
The planes (dunno, are those Mustangs/Wild Cats?), tanks (either a Wolverine or Hellcat Tank Destroyer) and the soldiers (US Paratroopers with full D-Day parachute equipment/uniform) are straight from the D-Day and the following days after the Allied landing, so the ruined city and the seaside location has to be somewhere on the coast of Normandy (fitting with the destruction and sirens).
Meanwhile Mads and Del Torro are clearly running around in modern clothing, likely from a near future setting (Mads M4 Assault Rifle with rather futuristic attachements and Toro's "Bridges - Free Cities of America" badge), not unlike MGS4.
Extremely hyped, I always loved horror and especially the kind that involves mutation and corruption of humans and machines into undead, parasitic versions that only want to wipe everyone out or infect them, Sidonia no Kishi/Knights of Sidonia or Flood style. This fills the bill nicely. Seeing how the skeleton paratroopers interacted with each other in such a humanlike way as if they were still alive (with one putting his arm on the undead trooper kneeling in front of him to stabilize each other while covering the tunnels) is extremely eerie and creepy.