I was playing L4D2 last night and came up with this idea. You've fought hordes with conveniences of the modern world--pump action shotguns, IV injections, and pipe bombs. But what if you were a hale and hearty band of rogue adventurers, lifted out of the pages of Tolkien and Goodkind. You're all alone, you're in a strange time and place, and there are a thousand zombies coming to kill you. It's "Thou Lefteth 4 Deadeth".
Basically, it makes all the characters more like an RPG party with classes and differing skills. You don't even need to make new levels or anything -- it's all character-based fun. Let's take a look at the cast.
The Knight - Exactly what it says on the tin. The knight just has a sword for a weapon, no guns. But it is a big sword. It acts like a katana, but has more range. Maybe it can be skinned to look more like a bastard sword. Or Excalibur. Also, he has more health, since he's in armor, but (maybe) he's a bit slower. The thing I haven't figured out is what to do when he's incapacitated, since he doesn't have a gun, and a melee weapon wouldn't make sense. Maybe he can emit some kind of flash grenade-y type thing that blows all the zombies back.
The Sorceror - Also what it says on the tin. You can't have a knight without a mage to back him up. The only problem is, I think this guy might end up being too powerful, unless he's tested meticulously. He uses magic, and has infinite ammo, but his magic has a recharge time, maybe like the melee counter. The more you use it, the tougher it gets to use.
I would imagine that he has a fireball attack as his primary weapon, or something like that. I doubt there's code to make chain lightning or freeze enemies or things like that. But you could make him explode zombies or ignite them. There's a lot of other stuff you could make him do: spread fire everywhere, make an explosion, throw spitter goo, turn invisible (useful for fighting a tank), repel a horde (opposite of bile bomb). It'd be cool if he could surround himself with fire when he's incapacitated, but this makes it difficult for survivors to get to him. I think the thing about a sorceror is that he's got a lot of power, but it can backfire on him. It can get out of control.
The Defender - Instead of a weapon, this guy has a shield. His primary attack is the shove, but this does more damage and has no fatigue timer. He can also drill through hordes like a Charger (but it stops after a while--he doesn't run into a wall and get knocked silly). This knocks them all back and clears a path for the survivors. He's very receptive to adrenaline (maybe it lasts longer with him). And maybe he can leap higher. When he's incapacitated, he can hold his shield to defend himself and takes much less damage this way, but cannot be revived until he drops the shield.
The Archer/Healer - His primary weapon is a bow and arrow (or maybe a crossbow for coolness++). Now obviously this weapon is probably the farthest removed from L4D2's current tech, so it would need the most development. It's a little slower and is a projectile (as opposed to the bullets which are instantaneous damage). It works kind of like a sniper rifle--you can use a scope, and it has a laser sight (unlike the Sorceror, who does not). Maybe you can fire different types of arrows: acid arrows, exploding arrows, bile bomb arrows. It's not the best weapon in the game for killing massive amounts of people. Probably on par with a pistol or two.
But what makes this guy stand out is that he's the only one that can hold first-aid kits, defibulators, pills, or any health items. And he can hold a bunch of them, maybe an infinite number. But he can't hold anything offensive--no throwables, no ammo caches. His only weapon is the arrow. He's responsible for keeping the party healed, and can buff them with adrenaline when needed (or some other stat). Do not let this guy die, or it will be very hard for the rest of the group until you get to a closet or safe house. It also encourages the group to stay together. My biggest beef is that this might make the game into a long escort mission. He needs something to help him survive. Maybe he's more agile.
Of course, with each character having their own weapon, most of which have no ammunition, this makes picking up weapons and ammo a defunct feature. This takes out one of the game dynamics, but of course, I've put in so much more...
If anyone wants to take this idea and run with it, please feel free to do so. No credit needed, no questions asked. I know nothing about video game development (though I am a programmer) or the Left 4 Dead codebase. So I can't help, unless you want a beta tester. The only thing I ask is that when you're done, let me know about it so I can play it. Or don't. It doesn't matter.