I was working on this from since before the Back 4 Blood announcement, because this is the kind of thing I do for fun. Felt like now would be as good a time as ever to drop this onto the internet for perusal. Contact at @lumsell on twitter.

----------------------

SAWN-OFF

Sidearm

2 quick shotgun shots followed by a reload

Provides excellent burst for a secondary, mitigated by the long reload time

I'm pretty sure it's illegal to make a zombie game without a sawn-off in it.

ANTIQUE RIFLE

Tier 1 sniper

Winchester rifle

Works kinda like the Scout from the german pack.

Gives a long range option for the early game.

RIOT SHOTGUN

Tier 2 shotgun

A black shotgun with a handle on the front and a drum mag.

Faster (almost double) fire rate but lower damage than other autoshotguns. Fires full auto when the button is held down.

Clip based reload

Functions as a sort of hybrid between the shotgun and the assault rifle.

BULLPUP COMPACT

Tier 2 super SMG

It's a P90

Has the high fire rate of the tier 1 smgs with the damage output of the assault rifles, with a large clip to boot.

ELEPHANT GUN

Tier 2 bolt action rifle. Resembles an AWP, but with a hunter's camo pattern. Presumably the weapon of choice of a particularly sadistic outdoorsman.

Shots not only deal massive damage along the bullet's path, they also create a shockwave around the bullet that kills commons. This is not at all realistic but it is very cool, and allows the user to take out huge chunks of a horde, a bullet at a time.

FLAMETHROWER

Tier 3 weapon

A jury-rigged flamethrower crafted by some enterprising survivor.

Fires a dangerous plume of flame

Gets hotter the longer you fire it for. For the first few moments, it doesn't even set commons on fire.

CROSSBOW

Tier 3 weapon

Bolts are soaked with bile, compelling commons to attack whoever you hit

Replaces the bile bomb

[The infighting aspect of the bile bomb was never as fun as it seemed like it might be to use, so the idea here is to have a somewhat weaker but more consistent way of getting zombies to kill each other.]

RIOT SHIELD

Special sidearm, like the chainsaw

The riot shield protects against all damage from the front. It has a limited amount of health before it breaks.

LIGHT CARBINE

An MP5. Replaces the silenced smg but retains its stats, which otherwise had little identity to distinguish itself from the uzi.

MAGNUM

Now a revolver to visually distinguish it from regular pistols

PUMP SHOTGUN

Now has a shorter stock and barrel to visually distinguish it from the chrome shotgun

----------------------

MELEE WEAPONS

Subject to melee fatigue now, because in L4D2 their status as the only weapon type that doesn't require a reload made them disproportionately powerful

When ancapped with a melee weapon your primary attack is replaced with the melee shove, allowing you to try and impotently beat off the horde with the weapon you have on hand. No more pulling pistols out yer arse.

SMALL MELEE WEAPONS: Meat cleaver, utility knife, claw hammer, wrench and (rarely) brass knuckles

These have a much tighter swing arc than the other weapons, so they can't hit as many zombies in one attack, and they deal less damage to special infected, but they have a higher swing rate.

AMMO UPGRADES

Ammo packs now have three uses per survivor and an even shorter deploy time that does not interrupt movement. This makes them better suited for tactical deployment; the short deploy time rewards quick thinking.

Explosive ammo is adjusted; it has more blast radius, less knockback. Shotguns now fire explosive pellets instead of slugs even though that makes no sense.

Explosive ammo has new effects to make its blast radius more clear

New special ammo: HIGH VELOCITY AMMUNITION

HV ammo has improved piercing capabilities and accuracy. It effectively replaces laser sights.

PRIMARY WEAPONS

Only one per drop, instead of four. Weapons are more numerous to accommodate this.

Multi weapon pickups made sense for L4D1 where there was only one weapon per class, but now that there is more variety using guns other than the AK-47 should be encouraged.

In addition, this makes it easier to make sure your team is using a good spread of weapons instead of everyone going combat shotty again.

Dropped weapons have dimmer and darker outlines depending on how much reserve ammo is left in them.

----------------------

SWIMMING

New water mechanic

Survivors can swim on the surface of the water, but cannot attack while doing so

Swimming works like hanging off a ledge in that doing it for too long leads to you dying

Forces survivors to take a moment to cover their pals as they move through dangerous territory

Spitter goo floats

HEALTH

While in the incap bleedout state, your health is slightly maroon.

When on black-and-white health, your health bar turns grey. This alows your allies to identify when you're up shit creek, so to speak, and assist you in times of need.

Between 40-60 health, your health turns a slightly chartreusier shade of green.

FLASHLIGHTS

Allied flashlights are now visible to you (did you know they weren't before?)

This gives a few advantages:

Basic realism

Gives a laser sight style effect that makes it easier to see where allies are aiming

Allows your allies to help you see in dark environments

Allied flashlights are a little dimmer than yours are on your screen, mainly to keep things from getting too visually busy.

----------------------

ARMOUR PACK

Takes up the medkit/backpack slot

The armour pack grants about 75 points of armour health

Armour health can go above the 100 health maximum

When above 20 health, armour will slow you down to that level.

Armour removes the aimpunch from getting hit by zombies, grants immunity to (most) stumbles, reduces knockback, and allows for faster recoveries from knockdown effects.

Armour also grants some degree of pin immunity. Resisted hunter and jockey pins will cause a stumble, chargers will bowl the survivor over, vampires will instantly drop their targets, and smokers will still drag the survivor away, though that survivor will not be disabled. Resisted charger, hunter, vampire and jockey pins will deal significant impact damage, and smoker pulls deal slightly increased constriction damage.

The bright colours of armour packs make commons prefer to attack you over whatever allies are nearby.

While armour is equipped, it continues to take up the backpack slot. You can use the backpack slot again to remove the armour permanently. Attempting to pick up a backpack item while you have armor on instantly switches to the armor slot and begins removing it.

[The pin immunity might be a bit much, but the core concept remains: an alternative to the health kit that offers additional benefits in exchange for a loss of mobility or something similar, requiring tactical deployment to gain its boons. This encourages players to use the kit *before* they become injured instead of *after*.]

FLASHBANG

Takes up the grenade slot

Overloads the infected's sensitive hearing.

Stuns all infected in its LoS for a few seconds, including specials and bosses.

Bosses are stunned for a much shorter period of time than the others are.

Huge radius. Frees trapped survivors. Can pierce thin walls (Don't think about it too hard).

Startles the witch.

[All the other grenades are effective against commons but have no effect on specials. This one is specifically set up to buck that trend. If things get too hairy, be it because a survivor is pinned in a difficult location to reach or multiple survivors are pinned at once, this can be used to unfuck your situation rapidly. In addition, like the molotov, you can use it for some extra value against bosses should you so desire.]

SMELLING SALTS

Takes up the medicine slot.

After using, cannot be incapped for 15 or so seconds. Health can still enter the bleeding out ancap state, but you won't fall over.

Usable when downed by pressing mouse2.

If you use pills, adrenaline, or a medkit while in the bleedout state, you will gain 30 additional temporary health and return to normal.

Salts do not improve movement speed at all, so your movement while in bleedout will be at 1hp levels.

If the salts run out when in the bleedout state, your movement will slow down to nothing and you'll fall back down again.

[Salts allow for a survivor to make themself useful in times of intense trouble where they would otherwise be completely disposed of, at the expense of not providing any additional health like pills do. So as not to completely trivialise the teamwork element of incaps, these do not quite allow a survivor to revive themselves; since they still enter the bleedout state they will often require either healing or a rescue before the salts wear off]

----------------------

COMMON INFECTED

The 30-common limit from prior games should ideally be replaced with something a little more dynamic, so as to prevent the shenanigans with exploiting common limits that were present in high level L4D play.

CROWDS

Commons can now form large, dense crowds that spawn in the survivor's path.

Crowds are essentially stationary hordes. They consist of a whole bunch of infected that stumble about aimlessly until they see survivors.

Crowds exist as a) mini-horde events and b) to discourage rushing; survivors who attempt to rush through a crowd will rapidly aggro infected at all sides and find themselves mobbed.

CRAWLERS

Sometimes, a zombie can survive getting a leg shot off. If this happens, they begin clambering along the ground on all fours (all threes?) like a mudman from swamp fever. This helps to avert the minor issue where common's legs are invulnerable before they run out of health.

ELITE COMMONS

New types of common infected that make up part of the hordes. These basically exist to give a little more variety to the standard common infected shooting gameplay, which can otherwise grow a little repetitive.

BRUTE

Commons mutating into chargers or tanks

Slower, chunkier guys that have a lower movespeed but more health. Can't be knocked back with a melee strike. Adds some unique threat evaluation (faster, more pressing threat vs. slower but more eventually dangerous threat) to the standard common shooting gameplay.

HOPPER

Commons mutating into hunters or vampires

Lithe, quick bastards that jump around a lot. Adds some trickier targets to the standard common shooting gameplay.

----------------------

SIREN

New special infected. A tall, emaciated woman with a waterlike grace.

First she sings, summoning commons to her position. Then, when she has enough, she sends what zomboes she's accumulated toward the survivors in a thick stream.

While singing, the siren has reduced movespeed

The Siren requires line of sight to the survivors to use her zombie stream. If the survivors aren't in visible range, the attack will fail.

The Siren's movement speed is reduced while singing.

She is quite effective at ambush tactics, as she can summon infected while standing in a position the survivors are likely to be.

While singing, the siren creates a distortion effect that can be seen lightly through walls.

When the siren is killed, the commons around her are confused and instantly deaggro.

[This zomb would A) provide a new style of gameplay, who compels survivors to break formation to seek her out and B) gives the zombie players in versus mode an opportunity to control the horde, which would be fun. The slow movement the siren is capable of while she sings means she can turn a corner as she charges, so survivors need to stay on their toes when trying to find her before she attacks.]

RUMBLER

New special infected

Has natural armour made from disgusting bone growths protecting every part except the back.

Armour can absorb an absurd amount of damage, in the ~1500 range, but the weak point on the back will kill a rumbler in 150 damage.

Approaches the survivors and, once in range, unleashes a powerful ground slam, which creates a stunning shockwave. This attack deals a significant amount of damage and knocks you on your ass. The groundslam ensures that survivors can't just sprint past a rumbler to get a back shot, they have to properly circle around her while respecting her space.

While being shot, the Rumbler has her movement slowed, encouraging survivors to keep their attention on her.

The Rumbler accomplishes two things: first, she forces a change of strategy from the survivors by requiring them to either flank or focus fire her. The second is she creates a danger zone surrounding her that survivors must respect. While other specials can force the survivors to change their behaviour with a successful attack, the Rumbler forces survivors to split up and flank a target to defeat it at all.

JOCKEY

Survivors can now shoot while being ridden by a jockey.

Weapons cannot be switched while ridden. If the survivor is holding a grenade, medicine or a backpack item when ridden, they switch to their primary weapon when the jockey snags them.

Survivor's ability to resist the jockey's movement is severely reduced

Limits the survivor's overall turn rate when ridden. The survivor can still look around as normal in a cone in front of them, but attempting to turn further than that is slower.

[Jockeys don't have a great identity of their own; they kind of feel like hunters but small and annoying. This change empowers their ability to split up a team while reducing their ability to disable survivors completely, giving their own unique feel distinct from the other specials.]

SPITTER

Spitter goo gradually expands over its entire duration, slowly encroaching on a huge area.

Spitter goo is a little less threatening at the start of the attack

[This pushes the spitter's capabilities a little more, allowing her to cordon off large areas of an arena, further forcing the survivors to change tactics when she appears.]

VERSUS MODE

All "pinning" infected can now dismount at will(???)

When dismounting, hunters do a flip off of their target.

After dismounting, hunters cannot pounce for ten seconds.

The cooldowns for infected's abilities are reduced across the board, to reward players for staying alive longer.

[Infected are known to be a mite too weak in versus mode.]

Instead of being randomly assigned an infected to play as when you respawn, you instead get a choice of three random special infected. When you choose an infected, you are not offered that choice again for your next ~two lives.

Backpack healing items (health kits, defibs and armour) only spawn in the safe room, and additional armour and defibs spawn in comparison to campaign mode. Once you leave the safe room, the backpack items inside it vanish (for you, anyway). If you manage to bring one of these to the end safe room, you gain a score bonus.

----------------------

BOSSES

Instead of only having tanks as the "boss infected", the director chooses from a few options when the time comes to throw a big bastard at the survivors. Because only fighting tanks can get old.

Tanks still have the most health out of all the bosses, making them unique in that respect. Tank punches remain one of the most damaging moves in the game.

VAMPIRE

A batlike lanky bastard with the unique ability to fly, using a membrane stretched over his arms.

His primary attack is the swoop, which slashes the shit of survivors in a line. Swoops are essentially air-based charges, like a charger but with a tight turning radius that allows for some adjustment while in flight. The swoop requires a short startup time to activate, which makes it less effective at extremely close range. If the vamp swoops into a wall, he can rebound off of it to begin another swoop, like a wall jump. This can be devastating in enclosed spaces, so survivors are encouraged to fight the vamp outside.

The vampire can also pick up a survivor (or car) with his talons and then drop them from a great height. The vampire begins ascending slowly while lifting, but picks up vertical speed rapidly.

When lifted, a survivor can be freed with a melee attack, provided they are freed before the vamp flies out of range.

BANSHEE

A bloated wretch with a powerful set of air sacs.

Fires charged sonic blasts. These blasts have a large blast radius and can punt cars. They deal chunks of damage and launch survivors, like a tank punch. When charging, the blast cannot be re-aimed, so the banshee must pick a location to aim at before she begins her attack. A keen eyed survivor can look at the Banshee to figure out where she's aiming and avoid the blast.

The banshee can also fire an uncharged shot. This does no damage, but can stumble a survivor.

In addition to her sonic attacks, the Banshee is quite mobile and fast. She can jump up real high to gain the high ground. To balance this, her range has a limit, and she can only charge shots when on the ground.

TURNCOAT

A fallen survivor, loaded and weighed down by tumor-like growths. The turncoat is unique in that they retain enough of their mental faculties to remember how to shoot a gun, but not enough to not try to kill the survivors.

Essentially a Church Guy on legs. As they fight, they ramble to themselves, but they're too far gone to even piece together a single coherent sentence.

Fires upon the survivors with an assault rifle variant.

Chunky, but not all that fast. Their slow movement is counterbalanced by the massive danger zone their gun creates.

Aim is shit when moving but stabilises when they stand still. This forces the Turncoat to give the survivors an effective warning of his or her approach before firing.

----------------------