As a new member I will firstly introduce myself:
My name is Orlando. Mainly a console gamer, though I've played a lot on a PC.
Personally I do not have any experience in the game industry, but a few friends of mine do. I've learned a lot from them, mainly focusing on visual and sound engineering.
Even though I have no experience, I am a (graphic)designer with years of drawing experience. Concept art is something I love to do, but have not done it in any professional way.

:Disclaimer:
Before posting I did read a lot of topics on this website, familiarizing on the writing style, but I'm not sure if it worked out. This post is a start on a design document with a lot but not nearly all of topics covered. I would love to get some feedback on what I have so far.

I tried to elaborate as much as I saw fit on all of the topics posted.
Because of my visual mindset I have used examples with this in mind.

I have chosen zombies because I really love the idea of a apocalypse of this kind. But I believe this concept could also be realized with dinosaurs, except for the story part and the zombie types further down this post.

This idea is holding me in it's grip for quite some time, like most of you posting your ideas I'm very passionate about this concept document, I rethought a lot of the things being described and after some feedback I came up with something I for one would love to see. If someone thinks the same, please don't hesitate, I am looking for people who would like to make it complete, though this is a hobby project ;)

Last thing I would like to mention. Sorry if this post is or seems to long. I wanted to be so clear as possible and give you the best view of the game I could imagine.

:Disclaimer:

World:
As I see it now the game could be best created in a sandbox world supported with hubs.
most of the world you are free to explore, though certain event like running-sequences (explained a bit further) could need a hub to support this feature.
Hubs could be an open room or section of the city / town the where story will take place, with borders created by cars blocking a path, buildings that are collapsed or another obstruction.
Outside of the hubs the world is free to explore and most of the story will take place in this sandbox like world.

Main focus and gameplay mechanics:
one of the first things I noticed by comparing movies to games or great games to games was the lack of awareness of the zombies, accept for Left 4 Dead. Movies like Zombieland addressed this feature greatly.
I imagined a world overrun by zombies totally aware of their surroundings, particular with sight and sound.
With this concept in mind I realized a stealth game, combined with high adrenaline action would be something I wanted to see.
Another concept I really love is choice, give players a choice that could effect the game greatly, though this choice would not necessarily be a choice between "Good" or "Bad", like Fable, nor will it be to choose whether a mission would be played or not, like Fallout, Infamous or prototype. I rather see unconscience choices being made, though it might include some of the previous mentioned choices.

To put this all into perspective:
Imagine a big square with buildings surrounding it. You are looking for (a) weapon(s) and there's a gun store near the end of this square. but this square is overrun by zombies. There is no way you can get by the store by running through the horde of zombies, even though they are slow, you are outnumbered. So stealth is the answer. barricades are scattered over the square, in the past they tried to stop the hordes, but failed and the survivors got overrun.
So you've got the materials to sneak by the zombies and reach the gun store.
After some stealth, you'll notice the store being closed of with the gates down, for who will let such a store be unguarded?
The owner lives upstairs in the same building and you figure, he should have a key somewhere.
Again you need to sneak in an ally to get to his front door. You walk upstairs, but the apartment is empty.
After a quick inspection you notice a key box locked. and a knife, or something down the lines of a crowbar which could be used to lever the box to open.
now you get to chose: Either you try to open the box without the tools, brute force would be the keyword, but then the zombies will hear you and will come after you forcing you to escape through the roof. Or you use the tools which and you will enter a mini game, trying to pry open the lock. Be to hasty with the mini game and the zombies will hear you, you need to escape, do it subtlety and the key will be yours. When walking downstairs toward the entrance of the store, the player will be awarded with a cutscene showing the protagonist enter the store safely.

If the entire scene forces you to escape you could never get the particular gun(s) again. In this case the downside could be that certain fights / items / events are harder / harder to get.
But this choice system could easily change the course of the story, in case of letting someone survive or die for example.

Another aspect is the action, I love myself a good stealth game, but it could become a slow-pace game, even boring.
So to compensate this, some high intense action sequences could fill the void. Action could include but not limited by the following:

- Defend mission, a static or semi-static position of the player trying to defend a hide-out or important structure or person.
- a shoot-out, other survivors attack or defend while you need to escape, eliminate the threat, capture or give in.
Like mentioned, sound would normally be a big no-no, in some of these cases shooting could be a necessity letting you use the zombies be a tool for dispatching enemies or other annoyances, though this could as well be used against you. Except in the defending sequences.

- running sequences, the player triggered a horde of zombies running after him/her, obstacles need to be avoided, for example an Uncharted like sequence.
When not in running sequences, running is possible but stamina could be a factor. Walking stamina is full, running for a certain time will result in a depleted stamina. When stamina is depleted inner strength is activated. Inner strength will result in a 10 second sprint which is faster then running, but as a result, stamina will take a lot longer to fill. Visually this could be archived by turning the screen black and white.

Fighting:
For most of the time stealth will be the answer, but what happens after you are discovered? Fight or flight will be the answer, sometimes the only answer is fight.

Melee: Melee weapons are scattered throughout the world, but are to be considered a last resort. Physics are similar to the real-world, a corpse will have weight and tissue, the result of a blunt weapon is that it needs a hard swing to do significant damage. Quite impossible with a lot of zombies in the vicinity. Alternately a sharp weapon could end up getting stuck within bones. Zombies aren't stable walkers, them falling over and hitting you taking you off balance is another thing. Last, they function with brute force, like a child not knowing how strong it is, getting close is a great risk, getting hit to the ground with loads of corpses walking around could easily get you killed.

Rocks: Rocks can be found and used as a distraction, much like a grenade in shooters there is a limitation to which you can carry (5)

Guns: In total there will be 5 weapon (gun) types to be found in the world; Sniper rifles, shotguns, (semi)automatic rifles, handguns and tasters. But only three types can be carried at once. Unlike melee weapons, guns aren't scattered across and should be considered rare, same with ammo. And each weapon carried could en will likely change the course of the game.

at first, guns need training. i'm not sure if this should be done with a leveling system or just the amount of time shooting with a certain weapon / weapon type. If the latter will be used, this will mean that at first aim is of, visually this could be done by recoil. If you pull the trigger a gun will recoil which will result in more misses. Training could also be done in safe houses especially created to create a better aim / reload time.

Secondly, a rifle will make some fights easier simply by it's stopping power and bullets per minute. A taser could be used to interrogate and get certain intel, a shotgun will dispatch tougher enemies easier, a sniper could save certain NCP's used for intel or places to be reached faster or not being able to get the intel or go to places entirely.

Upgrades:
Throughout the world items can be found to enhance weapons, duct tape, rope, clips etc. but these enhancements will be reduced in quality when uses. duct tape and clip could result in faster reload time; a double clipped riffle. Rope can be used in the same way as duct tape but the enhancement will reduce in quality faster.
With these upgrades it is possible to change enhancements from certain weapons to weapons which usually doesn't have such an enhancement; a scope on a desert eagle for example. off course this could create a weapon undesirable. example: a handgun doesn't have the range of a sniper riffle rendering the scope useless in close combat or medium combat.
this could extent gameplay by experimenting and creating new types of weapons for certain situations.

Zombies:
Zombie type seems to be inevitable but I like to create a certain depth or reality in my opinion unlike games like Resident Evil or Left 4 Dead. A zombie type will have certain behaviors or abilities depending on the careers they had while being alive or the state of decomposing, zombies also communicate, if one finds you most of the times more will follow. Following the types I have already created:

Walker: Most common zombie, slow because of the legs, eater to many chunks have been bitten of or decomposed to much to move fast. but they bouncing side to side a lot. Every weapon type could kill this one but a steady one would be most appropriate.

Hunter: Very common, moves fast (running), most of its legs aren't decomposed or bitten off. Arms on the other hand are. Some of the hunters bones are sticking out of their arms resulting in sharp ends ready to kill.

Brute: Fairly common, a brute neither moves fast nor slow, it can run. A brute used to be a police or a soldier, with body armor. A shotgun or a weapon with armor piercing bullets are preferable, otherwise the back is the weak spot. Frontal attack is possible, but could result in ammo depletion, needs a lot of rounds.

Blinded: Blinded are like the names implies totally blind. These zombies rely on sounds, walk to fast, fire a gun or throwing rocks can activate there silent stance. Much like the witch from the Left 4 Dead these are to be found in empty spaces, unlike the witch from L4D these zombies don't make a sound and are only visual to spot. Also unlike the witch these zombies are easily to eliminate, but are always in packs, startle one and more will follow. Best choice of weapon would be one with a silencer. Otherwise it's best to ignore one. These run as well.
A taser for these type of zombies isn't an option. This will result in numerous zombies all over you.

UI:
Like Dead Space, I would like to see a minimum User Interface. Maybe not at all.

Ammo: Everytime a reload is used the character will whisper the amount of rounds left, for example 10 rounds left, will result in "10 rounds left, damn need to find more". 5 rounds left could result in: "5, better make 'em count". This could be accentuated by subtitles or in just the amount flashing in screen. So reloading in a intense scene could be your end if a zombie will hear you. It could be optional to see the amount of ammo in a traditional fashion.

Health: The more damage you take, the more the screen fades. Like most shooters and adventure games these days. Health will recover automatically.

Inventory / Weapons: Another way to display ammo could be within an inventory. A quick select tool to select weapons is an option, though this could make a sound, the 3 weapons are on or around your body. A riffle hanging over your shoulders, when you duck the character grabs it so it doesn't make a sound. A handgun in your belt etc. If you change weapons it could make a sound.
The inventory could be in a similar fashion but when pressing a button the character takes of his/her backpack. Creating another opportunity to make a sound.
Distance could make a difference or otherwise it wouldn't be wise to switch weapons or check inventory.

Story:
For the story I have a few ideas for the main story, mainly beginning and end.
A lot of the story flows from the gameplay and focus of the game, hopefully creating a solid experience.

A huge chunk of the game will focus on curing the zombies, you were one of many responsible, killing most, saving few, depending on the state of decomposing.
Finding ingredients for this cure, some of the time with the result of doing favors for NPC's.
After the supposed cure is made, you need test subjects. But all experiments fail. There is no cure.
With the end of humanity in mind you will try to make a last stand by creating a settlement. NPC's will either be with or against you and depending on the choices made during the game the ending will vary.
examples:
- You let one NPC with an agenda against you alive or you didn't help him in a proper way, he/she rounds up more who will try to overrun you, leading to casualties, your settlement dies out.
- Through some choices the zombies eventually find you safe-house, being overrun by zombies humanity's hope is lost
- You help most important NPC's the way they like, your settlement flourishes. More and More people join, rescuing humanities fate.
- You've convinced most NPC's to join, but zombies find your settlement forcing you to evacuate to another location, maybe in the form of a deserted island.
Etc. Etc.

By the choices made during the game, by different gamers, every game tends to have his own story with it's own outcome.
By using the choice system uniform, not every choice needs to have an impact that will create a different outcome but will pose to players, just as important as the choices that do. This will hopefully create a high replay value.

If someone would like to help me complete this document, please don't hesitate to PM me I would love to see this come to life!

This all seems incredibly ambitious for a first project. I would recommend scaling back and focusing on one or perhaps two core elements that you think are the most important, and working on those. To make a game similar to what you're suggesting, you'll need 3 years and ten million dollars.

However, I'm not just dismissing these ideas. I love the idea of stealth and zombies and don't think I've ever played a game where they've been successfully married together before, so I'd certainly love that aspect. I also find that in a stealth game with horror elements you have the inner dichotomy of wanting to stick to the shadows, as they're where you're safest, but also being totally spooked out by all the shadows. I sometimes get that when I play Skyrim and some of the spookier music plays, or in the somewhat older Thief series.

I think the action oriented stuff sounds a little old-hat to me, and really does a lot to detract from the spookiness and scariness of a stealth game. In Thief I remember you could use flame arrows to take out a guard (noisily) in one shot, but it was always more fun to sneak around and club them over the back of the head. Perhaps limit the player (apart from in some set piece situations) to silent melee weapons on zombies? That way taking a zombie out becomes a real challenge and gets your heart racing every time (the traditional thinking being to stay as far away from the dreadful things as possible at all times).

Alternatively, never underestimate the value of doing it 2D. This could work well for a top-down game and would drastically simplify the art requirements - you could use your concept art experience to create some really lavish worlds and characters, and the math becomes a lot easier in 2D too.

A lot of good ideas here, really too many good ideas

Simplify and focus. What is your core? It appears to be stealth and zombies. That is a unique twist on a tried and true genre.

Begin there - what does it mean to marry stealth and zombies? Minimize everything else and concentrate on this core. Get the core right and then do everything else. Develop prototypical fights where stealth is needed. Perhaps stealth is used to disable zombies, or distract them or just to sneak by groups of them. Whatever it is concentrate on developing this first. I guarantee you what you start with is not what you will end with. Issues will come up, things you thought would be fun really aren't and new inspiration will strike.

I guess what I am saying is don't worry about the 80% of the game which is NOT new (UI, weapons, inventory, health model, etc). Concentrate on the 20% which is new and different and unknown - the zombie/stealth idea.