> Video game concept
> Class-based zombie apocalypse sim
> Approximately 95% of the world's population has turned into zombies, either by being infected with the virus or by coming into contact with something that has. Sound familiar? Of course it does. Unlike other zombie games though, mine would be bigger, and give you (the player) much more freedom than, say, L4D. You play as one of the few survivors in some of the most famous cities and towns across the globe. New York, Las Vegas, Miami (zombies in Disney World, anyone?), Tokyo, Moscow, Berlin, Hong Kong, Sydney, and plenty of places in between. And it's not just going from point A to point B either. Your goal is whatever you want it to be; mere survival, protecting other survivors, or maybe even taking the fight to the zombies, killing as many as you can. Whatever it is you want to do, your past experience will come in handy in some way. You can create and customize your own character, including his/her clothes. You also choose your character's former occupation, which decides what special abilities you get. I haven't thought of all possible jobs and what skills they would have, but a few of them are:

Doctor -> Heal/buff allies.
Criminal -> Pick locks, hotwire cars, etc.
Engineer -> Build better barricades, build traps, fix stuff faster.
Athelete -> Better mobility and higher physical strength, allowing you to move/carry heavier objects easier.
Police -> Better gun handling, i.e. slightly faster reload speed, slightly more accurate.
Pilot -> Able to fly/drive any vehicle, and is better at driving cars and motorcycles than other classes.
Firefighter -> Higher damage with melee and incendiary weapons, takes less damage from fire.
Biologist -> Higher damage when hitting weak points, can dispel various chemical attacks, like Marker scent.
Cook -> Lose stamina at a slower rate, and gain special buffs from food. Also causes you and everyone around you to gain more stamina from eating than normal.

That's all I've got so far. There would be some RPG elements in it as well, where you get a little bit tougher, faster, stronger, and better at what you do as you progress. In addition, the weapon system would work on a looting system. You have to find them if you want to use them, whether it's from a gun store, a police station, or someone's house. Once you have it though it's yours for as long as you want to carry it. You can even customize it with custom paint jobs and attachments, but you have to find the attachments as well. You also have to eat, or your stamina will drop, resulting in you not being to run as far, or as fast, and your accuracy drops a tiny bit as well.

With the emphasis on teamwork and classes, the game would probably be best as a sort of mini-MMO. Something like 50 players per city should be managable, since it's not PvP. Heck, maybe even 100. Even then with the size of the cities a lot of players would likely not find each other.

Now then, on to the zombies themselves. Like L4D, there are more than just your average shuffling skull-munchers. The virus affects certain people and animals in different ways, resulting in strange and dangerous mutations. Again, this is one part I don't have thoroughly fleshed out yet (lolpun).

Cerberus -> Unlike most zombie dogs, this one has grown to the size of a minivan, and has a mass of fang-covered tentacles where its head once was. This mutation makes it less effective than normal zombie dogs at finding survivors, but it is much more dangerous of an opponent. Not only does it lack the normal weakspot of a "head," but it can use its swarming tentacles to attack multiple survivors at once, and even attack around and through some obstacles. Sure it can't fit through most doorways, but that doesn't mean that it can't grab you through a window.

Nuke -> This zombie is little more than a bomb with legs, and wants nothing more than to clear the way for its hungry brethren. It normally hangs back and stays out of the way until it is needed to destroy a barrier or a vehicle or a particularly hard-to-eat survivor, but when it is called forth it runs at you, screaming and already beginning to split at the seams with explosive energy. If you can destroy its head it will lose its sense of direction and may veer off course, and if you destroy its legs it will fall down where it stands and explode, often taking other zombies with it.

Spider -> Not all zombies are limited to walking the streets. Spiders' limbs have split into several spider-like legs, allowing them to climb up and down anything from trees to buildings. Rather quietly too. Often, the only warning you'll get that one is about to pounce down on you is their trademark hissing. These zombies also have a tendency to create thick, slimey threads all across their territory. The longer one is left alive in an area, the more of these threads you will find. These act as bridges for the Spiders, as well a way to slow down survivors. Luckily for us, the sticky ropes are highly flammable, as are the Spiders themselves.

Knight -> A walking tank of a zombie, this particular mutation causes the afflicted human's bones to grow and contort into a thick, tough exoskeleton. They also have one of their arms replaced by a large, lance-like spike of bone that they use to pierce through anything they feel needs a large hole in it. They are very hard to kill using conventional means, but are rather slow due to their heavy armor.

Raptor -> Most birds simply die when exposed to the virus. The ones that don't, whether they be pidgeons, hawks, parrots, or crows, become Raptors. Raptors are big, averaging about 7 feet in height, and can no longer fly, their wings now hanging limp and decaying. They now wander the cities gorging themselves on corpses, which are plentiful, obviously. But while rather fat, these monstrous birds still have excellent jumping ability, and are extremely aggressive. If they notice a survivor they will chase them until either they or their prey is dead.

Marker -> This is one of the newest and most insidious mutations of the zombie virus. It doesn't have any combat ability and is very fragile, but it is very unwise to let one get close to you. Markers are unique in that unlike most strains, they tend to show up alone, and in places where most zombies don't generally linger. Markers like to hang out inside buildings and other rather closed in areas, as it suits their purposes better. That's because their purpose is to locate and "mark" survivors so that other zombies can find them. If a Marker senses you it will immediately start to follow you and try to touch you. While this may look rather funny, with the zombie walking towards you with both arms outstretched, like in the old movies, it is a dangerous situation. A Marker's handprint leaves a unique scent that attracts zombies like flies to rotten meat. Be warned though, even killing them has its risks, as once killed they release a large cloud of their scent, marking everything and everyone caught in the blast. If you kill a Marker, you can be sure that zombies will soon flock to the spot where it died. Whether or not they will then be able to track you down depends on how close you were to the Marker when you killed it.

Herald -> A close cousin to the Marker, the Herald serves a very similar purpose. There are really only two noticable differences between the two strains. The first is that while Markers lurk around more enclosed areas and shamble towards you when they spot you, Heralds can be found anywhere, inside buildings and out, but still almost always alone, and instead of shambling they run at you, cackling like a madman as they do so. The second difference is that while Marker scent attracts regular zombies, Heralds attract only a certain few types, known as royals. In addition, their trademark laughter also attracts the royals, if they are very nearby. Their scent, though, will draw in a single royal from virtually anywhere in the city.

Jack -> One of the "royal" zombies, there are only a handful of Jacks in a city at any one time. They normally stick to rooftops, but will descend if called by a Herald. Jacks are able to fly with their bat-like wings, and can spit a powerful acid that can burn through even metal. Jacks are one of the few zombies you have to worry about when flying in a helicopter or jet, as they can fly quite fast and are very accurate with their acid spit. In addition, when on the ground their wings act like shields, able to deflect small caliber rounds and other weaker attacks.

Queen -> Another of the royals, Queens are easily identified by their lack of visible decay. They aren't bulbous or misshapen or covered in tentacles and spikes. What they are, however, is strong, fast, and smart. Strong enough to break through most barriers with ease and throw cars at you, fast enough to chase you down and be hard to hit, and smart enough take cover and even tactically retreat if she has to. If you fight one and she runs away, you can expect her to come back from a different angle accompanied by a group of minion zombies.

King -> While each of the royals is dangerous in their own way, Kings are easily the most visually frightening of the three. Always angry, always roaring, and averaging 16 feet tall with huge claws and a mouth big enough to swallow a cow whole, Kings look more like freakish mutant dinosaurs than they do humans. They, like Raptors, are extremely aggressive and will never stop once they've spotted their prey. They also have a trio of long, stretchy tentacles in place of a tongue, which they use to try to grab things that they cannot reach normally.

Cannon -> These slow, fat zombies have had their arms fused together into a sort of cannon-like appendage, which they use to fire hard, bony projectiles at high speed and with great accuracy. They can even shoot down aircraft. They are, however, very slow, and have a huge weak spot on their backs that can finish them off quickly. They are thus virtually harmless unless you are in a vehicle or stand still for too long for some reason.

Torch -> Torches are pyromaniacs taken to the extreme. These zombies run around randomly, and upon seeing a survivor will begin breathing fire left, right, and center, burning everything around them as they rush at you. Making them more dangerous is the fact that their arms have mutated into little more than movable shields which they use to protect their torso as they charge. You can destroy their head to stop their fire breathing, althoug the body will continue running for a while, or you can destroy their legs to make them fall, although they will continue belching flames. The safest way to dispose of them is from a distance with explosives, which, combined with the Torch's natural flammability, causes a large, fiery explosion.

Behemoth -> An incredibly rare strain of the zombie virus, Behemoths are not actually any single creature or person. Rather, Behemoths are made from one central zombie absorbing and fusing with other zombies. These massive piles of flesh and bone roam around the cities and will attack anything they find, zombie and survivor alike, which is why both generally flee as fast as they can. It is theorized that there is no limit to how large these things can grow, and the biggest so far was reported as being six stories tall. They aren't invincible though, as destroying the central core will cause the entire shuffling mass to break apart and die. The core can only be damaged by destroying the flailing limbs and outer shells that protect it though, and the bigger it is the more of both it will have.

Leviathan -> Leviathans are what happens when a whale miraculously lives through exposure to the zombie virus. Most animals simply die, whales included. Leviathans, however, don't just survive, they grow bigger and meaner than any whale has a right to. Only found near the coastal cities, obviously, these gigantic creatures are no threat as long as you stay on dry land or in the shallows, but any boat or swimmer that goes out too far is doomed to be a meal for these giants.