Maybe it's the blood loss from my nosebleeds and lack of sleep.. But I think this is an exceptionally cool idea. To take the basic plots of either Duke Nukem (aliens invade earth, leaving only a select few humanoid type races to choose from while others are alien scum..) or DOOM (You journey to hell, but you don't know it's hell yet..) and take on monsters.
To fully get the feel of the gritty mechanics. I had some ideas that would totally change 4th edition. Some might complain it'll hurt game balance, but my intent is to make things random and thus challenge player's minds and stratagy..
Stats:
Instead of point-buy. Dice roll, this isn't uncommon, but can definitely make things a struggle. Roll 2d6+6 six times for stats.
HP:
In fourth edition we do not roll for HP. Now we do. I'd have to deeply go over by looking at the classes how much HP should be rolled. But I'm guessing something like Fighter: 2d10+ con mod at first level, 1d10+con mod each additional evel. All characters have starting HP equal to con score.
Power Points:
Rather then "at-will", "encounter", and "per day" powers. Powers will come at the risk of depleting powerpoints when you activate them. But not automaticaly! I'd say to roll 2d100, and try to roll equal to or above the power's level x2 to activate it without powerpoint cost on both. (So anywhere from 2 to 60, but you have to roll succesfully on both for it to count) Then, if it does cost powerpoints, you have to roll a dice to see how much is depleted.
Level 1 to 10 powers=2d4 pp
Level 11 to 20 powers=2d8 pp
Level 21 to 30 powers=2d12 pp
Like HP, you roll each level for powerpoints, and different classes will have different amounts of powerpoints.
Powers Known:
I forgot this almost. But you'd know more powers. It's recommended that you begin the game at first level. But if above, change the die roll if any. But generaly a first level campaign would have you begin knowing 1d8+1 number of powers. (You might be able to learn all your powers!) This sortof versatility is encouraged. And infact should be explored in-game as rewards for good RPing. Rather then simply action points, a PC may be gifted with knowledge in a new power for that appropiate level.
Feats:
As with powers, you can roll for more now. Roll 1d4 for starting feats. Generally a character would only start with one feat. So this is going to improve them. This has another value too. To make characters fair. Some might roll high on stats or powers known, other on HP or powerpoints, some might on feats. It's doubtful anyone will roll high or low on all of them. The extra feats will be good too sense you'll want them to learn new weapons.
No Healing Surges!
This is a big one. Healing surges are not replaced with "healing orbs" which can sometimes be found across levels, or when you kill enemies. Healing orbs can cure HP, PP, or both. And usually involve rolling dice to see just how much.
Weapons:
Though not as bad as in third edition. Weapons are un-even and especially in this type of game where people will be "finding" more then "buying" gear. (And gear will get destroyed ALOT..) They're going to have to be relatively even. (Though a little un-even can be good. A little in-game conflict for the BFG might be amusing..) As such, several new stats have been added to weapons. I'm going to use The Maul as an example, as that seems to be one of the visibly most underpowered weapons..
The Maul
Damage: 2d6 Prof: +2 Confirmation: 16 Massive Damage: 15 DC: 3
What does all this mean? Well, let me break it down for you.
Damage is self explanatory. Prof is how much bonus attack you get for knowing how to wield that weapon properly. Both of these were already in fourth edition..
Confirmation is a seperate d20 from your normal attack roll. The attack has to succeed inorder to confirm. Unlike before, where a "natural 20" on attack would deal maximum damage. Now it only establishes you hit the target regardless of AC. The confirmation roll now determines if maximum damage is dealt. This all-in-all means that you'll be dealing more damage.
Massive damage is a seperate d20 from your normal attack roll much like confirmation. Like confirmation, the attack has to be succesful inorder to do massive damage. When massive damage is dealt, your opponent has to make a saving throw DC equal to what the weapon states. (So the DC for the maul is 3, pretty low, but then he's got a 25% likelyhood of dealing it!)
If your opponent fails their massive damage saving throw. Damage that would be dealt to HP, is instead dealt to the monster's vitality. Vitality is a more abstract, "hidden" HP score equal to your constitution score. Much like Star Wars Revised Edition, these are your backup healthpoints or "toughness points". You can cure toughness points using a health orb, but for every 2hp you'd heal, you only heal 1 toughness point. Making it obvious you do not want to be brought down to relying on toughness..
Unlike PCs, all monsters only have a toughness score equal to their constitution modifier. (No less then 1) Unless you plan on using them as a boss or otherwise important NPC. It is encouraged for you to reduce it if this doesn't prove to be sastisfactory... There are two reasons for this:
A) To simplify combat.
B) To make the opponent less threatening then the PC. Obviously it wouldn't make much sense to be campaining in a game where the monster is every bit as powerful as you are.
Powers may or may not contain massive damage, though it's doubtful I'd implement them into powers. This might sound like the fighter will have a clear advantage using weapons but you forget, the fighter uses powers to use his weapons effectively.. And every class has different dice they roll for powerpoints.. The Fighter will become a striker in pratice because of this..
Obviously this will require some play testing. But I like where this is going. I also thought of revamping some monsters abilities to bringback save-or-sucks.
Bodak Gaze: (At will)
The bodak must be adjacement to the foe inorder to make a gaze attack. If caught in The Bodak's gaze, an opponent must make three saving throws. (DC: 6) If he fails all three, he dies, and spawns into a bodak 1d6+6 rounds later.
You should be able to make a DC of 6 on a d20 atleast 1/3 times. If you can't, then I say you deserve to die. And like any hardcore videogame. You're going to comeback as one of the enemy.
No Inniaitive:
I don't really think innaitive is nesscaray. I think it just slows down the game. As long as players act reasonably and don't hog the glory/powergame. They can take actions when they want. I would however, to keep dexterity a relevent stat, impose that along with needing the proficietcy of a weapon, need an amount of dexterity to use said weapon. Without the dexterity, it's treated as though they weren't proficient. Even if they are proficient. I put alot of emphasis on weapons. But then again.. it is suppose to be a first-person shooter..
That's all I can think of for now. Hopefully you'll like my idea for a campaign. Even if it completely sodomizes fourth edition lol!
My only regret is I couldn't think of a way to implement any more dice rolls... :P
EDIT: Some new powers have been added. These are homebrew and you don't have to put them in your game. Just something to consider incase you want to reward PCs or they ranout of powers to choose from. I didn't get into range and such, no need really. Use common sense.
Hexen Lightning:
Remember that lightning spell from hexen? It was toturtous. If you don't know why, you'll see..
Level 1 Power. Power Check: 4/100 (the same as any first level encounter power)
Attack: Intelligence vs Reflex.
Hit: 1d10 damage. Then the foe makes a saving throw (adding their dexterity mod) DC 20 for the next 1d4+1 rounds. Every round they fail to make the saving throw, they take half of the damage rolled on the d10.
Vegenful Souls:
Another Hexen classic. This one is based off the attack used when the cleric used it's cross. Causing souls that shrilled and swarmed the stage, damaging every foe on the screen.
Level 1 Power. Power Check: 4/100. (The same as a first level encounter power)
Attack: Wisdom vs Will.
Hit: All creatures that are hostile toward you with the radius the vengeful souls can reach are immbolized untill the end of your next turn.
Volcanic Hammer:
Yep, taken from Hexen, this is one of the fighter's attacks. It was a slow moving but powerful fire hammer thing.
Level 1 Power. Power Check: 6/100. (The same as a first level daily power)
Requirement: Must be using a hammer weapon.
Attack: Strength Vs Fortitude
Hit: 1W+1d8+wis mod fire damage. The foe then makes a saving throw DC 13 or takes an additional 1d4 fire damage every round for the next 1d12 rounds.