My Perfect Zombie Game: you are a survivor in a world-wide Zombie outbreak and you live in a small sequestered village in what was once a thriving city, you were born into a world of zombies, you live in a world of zombies, you will die in a world of zombies. You find your own food, search for your own water, and eke out survival in a hellish world. One day, you hear a repeating message on the radio that you are never supposed to turn off; a set of coordinates, you don't know where but can find it in the old library across town. you and your band of 3 survivors decide to set off for the unknown location of the coordinates as you are running out of places to find food. you must find what you need along the way, all you have is your father's old rifle and 6 bullets, minus the ones for you and your group. the road will be harsh as there are hordes of Undead between you and the library, not to mention where the message leads. One bite and you will die, one wrong turn and you will be surrounded, one mistake and the only humans you know will perish. You must find what you need to survive: food, water, guns, ammo, tools, these are the things you need to survive. You don't know who sent the message or even where it leads, you can only follow it.
I didn't read any of the posts except the OP, so if my idea sounds like yours, FU its mine. and I barely know anything about Left 4 Dead other than I want to play the shit out of it so again, FU, MINE!
Fill in any details you want that I didn't say or to fix it this is just my preface.
The main mechanic that I wnat focus on is the survival mechanic, not to say that the fighting mechanic is subpar its just that it is a survival/horror game, so you do actually need food water and shelter every day and tools when the ones you find break or you are finding it for the first time. Since you are traveling your weight must be light 50 ibs min if you want to run 75, if you want to jog..etc feel free to change to fit the game.
And I use Romaro Rules to my Zombie games, No Zombie is distingguishable from another one in terms of mental capacity, etc. (No fucking smart Zombies either) this also save time as you only need one AI. Though you can have the fast zombies if you want as zombies are only as strong or as fast as their human counterparts.
There will be guns only as the day-saver but the fighting mainly comes down to melee. (your machete is your friend)
I was serious when I said one bite and you die (I'm tired of games that don't do it) the way you can incorporate that into your game is that zombies mainly try to grab and claw and grapple so they can bite you. So you can probably incorporate a button sequence when the zombie grabs you.
Group survival is a must, YOU HAVE TO KEEP YOUR GROUP ALIVE AND WELL. It will be almost impossible to solve the game without your group, so build the game around that concept. This also makes for fun Co-op and Solo achievements as well.
Since the Zombie is the main enemy in the game. I say main because there might be other humans going to the Source (message) and you are fighting for the same resources, not to mention brigands and thieves, you are going to focus on a reactive AI to keep the zombies interesting, I will leave that up to you.
Another concept you could incorporate is that the zombies learn collectively giving your enemy a face (I am willing to bend the Rules on this for the sake of fun.)
Also I want a full unlockable campaign as a zombie no half-assed "you get to play as a zombie in multiplayer," shit i want the full dev's attention to being the zombie, fuck the realease date.
This is just a rough draft that I thought up about midnite so e-mail me if you have any questions or stuff as my game can go on and on and on. firstname.lastname@example.org
The dev's objective in this game is to give gamers the one and only zombie survival expirience as modern and realistic as you can make while still being fun and original
Oh and as for multiplayer do what you want I play Solo.