Here's what I've found on the engine:
The project is internally named Zion.
The engine has a similar architecture to Doom 3's.
There's a new system named AAS2, which seems to be for collision and area data.
The CVar and Console Command systems seem to have been merged.
There's a command-line utility help string in here, mentioning Orbis (PS4 devkit) and Durango (XBox One devkit):
And there are a few occurrences of "crap" which I think is funny.
Now for gameplay:
There is a new weapon named the Mancubus Gland.
You can play as a Mancubus in MP (not available at the moment).
The Mancubus has a heat ventilation system in both SP and MP. It can kill you.
There are bounce pads.
There is a new enemy named the Hell Soldier, but is only referenced in two places. Perhaps removed.
There is a Codex, I assume this is like Doom 3's.
There are 6 keys. Maybe more.
can be resurrected, dash, strafe, charge attacks, use seeker missiles, evade your fire, use a gigantic scythe
, do a ground pound, use missile barrages, and choose between several ammunition types.
He also has several stages of damage, where his behaviour changes
Enemies have pain chances, and multiple states of pain.
These are (ordered): twitch, heavy twitch, interrupt, stun, falter, push back, stagger, and knockdown.
You can modify your BFG.
It has: Rapid Fire, Charge Attack, Beam Fire, Spherical Fire, Arc Fire, and Burn Fire.
Burn Fire is a flame BFG.
It also has mods for double damage and increased capacity.
There is also a new weapon named the Arc Cannon. I do not know anything other than it does splash damage and charges cumulatively.
Another confirmed weapon is the Lightning Gun, and Damage Shield.
The Revenant can punch you.
There is a 3-D automap. It is indeed named "automap".
There are 3 difficulties, with Ultra Violence and Nightmare being special modifiers to Hard.
The game has dynamic difficulty scaling, using some system named "AIC".
It seems similar enough to Resident Evil 4's, so you can just look at that if you want reference.
Ultra-Violence screws with the dynamic difficulty on Hard, making it really fucking hard.
Nightmare is ironman mode + Ultra-Violence.
Hell Knights can also wield scythes, like the Cyberdemon.
These scythes are named Hell Scythes, and are not physical. They have a life span, and when they die they explode into many projectiles.
They can be thrown, and they can be swung.
There's a weapon mod named Hell Shrine. Insufficient data to comment further.
Glory Kills can be disabled in PvP.
The PvP is UAC versus Marines.
There are unlockable player armor skins in MP.
There is a game mode called Hell Gate, where demons open portals to summon more demons and UAC must destroy these gates with bombs. The bombs can be shielded.
Another game mode is present named Hell Escape, which appears to be something like the game Left 4 Dead, but in more of an arena style.
There's also a Freeze Tag game mode.
Now, about demon classes:
There are a few new demons which can only be used in multiplayer. These are unique to the singleplayer demons.
The first one is the Harvester, who gains energy by shooting a beam at people and then detonating himself. I am unsure if this is exactly correct.
The second one is the Prowler, who makes creepy ambient sounds, and can perch himself somewhere to gain "Demon Vision" (which internally is referred to as a wallhack, no joke).
Other than that, he basically acts like the Hunter from the game Left 4 Dead.
For the story:
Phobos is referred to in a single internal string that just contains the name "Phobos" and nothing else. Deimos and Mars are not mentioned internally. There are no xrefs (usages) of this Phobos string, either.
And for modding:
SnapMap has a currency named snapCoins. These are not microtransaction bound, and are entirely just to keep you mapping.
There are a fucktonne of variables you can edit on everything ever in SnapMap.
There are modding tools named idStudio. These may not be available to the public immediately.
idStudio is very powerful, with its own scripting, action chaining, map tools, and much more.
Here's a list of monsters that are in the symbols:
Hellified Soldier (ZombieMan?)
Olivia's Guard (?)
Talisman Guard (?)
Cyberdemon (Hell) (?)
Hellified Shotgunner (ShotgunGuy?)
I previously mentioned an enemy named "Olivia's Guard".
This enemy has an unknown purpose, but has very interesting properties:
It can get into an (amusingly named) "tantrum".
They can select ammunition, so I assume their weapons use some sort of projectile. What kind of projectile they use is not mentioned.
They have a move named "Pool Ball".
In one string, they are referred to as "Olivia's Guards". Note the plural, this could be a recurring enemy and not just a boss.
For those who do not know, Abaddon is the Hebrew name of Apollyon. Apollyon is the Greek name of Abaddon.
This name refers to both a place -- a bottomless pit, a realm of the dead -- and a person -- a fallen angel.
They also have moves called "Recover", "Beam Attack", and "Hammer Enemy".
Abaddon and Apollyon appear to be weapons.
It is possible that Abaddon and Apollyon are actually living weapons, some of these strings refer to them as actual moving entities.
As for the Talisman Guard, I cannot extract any useful data. There are only a few strings mentioning it, which do not give any interesting information.
Likewise, Cyberdemon (Hell) is only referred to in a single string.