Hmm, let's see:
- NPCs, and competent, mostly-independent ones at that. You still got the strong-willed population armed with whatever firearm they could find (which differ from the stock weaponry; they cannot be dropped) that can hold off the hordes until the ships arrive. The ships pick up every last person on Earth about a third into the game, and you stay behind to contain the invasion yourself. By this point, instead of being action-packed and filled with story elements, it is eerily quiet and you basically figure out what to do with only a handful of hints available.
- Several Earth environments: a starport, a rural town, a highway, a suburbs, and then a city. The highway and the inner city have no people, only monsters.
- You have the BFG9000 from the original Doom, but it is somehow destroyed early in the game. You must find a secret lab in the city that can not only repair it, but upgrade it into the more powerful, somewhat different-looking BFG10K. 1 or 2 BFG10K's can still be found in Hell, or worse, wielded by a (rather weak) zombieman.
- In Hell, the portal is heavily guarded, but after that, demons are rare. Then, the further the player progresses, the more demons and ammo there is. Unfortunately, only ammo that the zombies and some demons use is available, and thus, there is no cell ammo, save for the dead marines scattered around that might supply some.
- Along with the M9 Pistol and BFG9000, you get an M16-like combat rifle. The rifle can be semi-auto, 3-round burst, or full-auto, and it could have a medium-powered scope attached to it.
- There are a multitude of cybernetic demons of different sizes and different weapons. Obviously, there's the classic Cyberdemon, which would stand 3 stories tall and has a good ol' rocket launcher and a claw/fireball attack. Then there are lesser cyberdemons that range from as tall as the player to 2 stories tall, sporting various attachments and weapons, such as a 4-barreled riot shotgun, a chaingun, a plasma gun, or hell, even a chainsaw.
- Obviously, the Baphomet final battle. However, you fight it a little bit in Hell after it just broke out of its recovery chamber (aka Icon of Sin), then it retreats and summons various monsters while doing so. The player then spots a closing portal and must fight his way through to it before it closes; if it does, then the game ends right there. If the player gets through, he gets back on Earth to find Baphomet there, staging a full-scale assault on the ships heading to Mars. The battle here is easier, as there is more ammo, and harder, as you must stop the assault on the ships before it begins. The battle takes quite a lot of ammo and a lot of health.
- Almost anything can be a melee weapon, granted it could be picked up and is hard and durable. Unfortunately, under berserk, said melee weapons break after one hit.
- Magazine reloading takes about a second, and shells are loaded 2 at a time. The rocket launcher, being full-auto like the classic one, normally does not reload due to ammo teleportation technology that works better with weapons that aren't rapid-fire (it hasn't been used in shotguns yet). This system actually consumes 1 cell per rocket; the launcher itself can hold 3 rounds, then manual reloading is required if there is no cell ammo.
- Health pickups heal gradually (probably around 10hp a second) instead of instantly. Like Duke Nukem 3D, certain interactions restore a small portion of health. Health can regenerate, but only at 1hp/second after 10 seconds when at 80+.
- Special >100 health pickups have no upper limit, but similar to Quake, the health degenerates to 100 at 1hp/second.
- The more armor points you have, the more it protects (e.g. 50ap = 25% protection, 100ap = 50% protection, 200ap = 75% protection). Also, over 100ap can slow you down.
- Speaking of speed, you run very fast by modern video game standards, and there is a stamina system similar to Doom 3. You can jump up ledges 5 feet tall and climb up 8 foot walls. You can go through vents when crouching, allowing you to get to securely-locked and protected areas.
- Only secure, bulletproof doors/rooms require keycards. You can destroy most normal locked doors.
- Some hub systems where appropriate, such as in large, complex buildings.
That's all the ideas I have for now.