Multiplayer survival game in an sandbox city. Games last about an hour in which you have to survive, stop the spread of the dead and develop the characters stories.
Each game begins with players in random locations as they see the zombies spread, attacking and turning the population of the city. The
players have to team up and find somewhere safe, then they can choose to get to the airport/harbour/motorway and escape (easy), or get to the military base/police station to try and fight (hard).
There's one city but there are multiple different types of zombies (virus, voodoo, Alien, nanobot, hell, etc), one type is selected at the start of each game. Each type of zombie has corresponding characters so one of the voodoo zombie characters would be a voodoo priest. Each type of zombie/character also has a back story which is developed through finding clues. It should take around 4-5 plays to complete each story and with 5 types there is around 25hrs play just to do the stories.
Players choose their character and then enter matchmaking which tries to get a good mix of different character types (skill level isn't relevant). The type of zombie is chosen by the game at the start by finding the story that has been developed the most by all characters. Any character can help develop any story by finding clues and sharing them with the others or helping to take down a key zombie (boss).
The different types of zombies have different characteristics such as speed, behaviour, how the infection spreads, how easy they are to kill and if headshots work. This makes it difficult for players at the start as they won't know which tactics are best and must play cautiously. Zombies will also mutate/develop the more a story progresses leading to boss fights or the zombies employing a degree of strategy.
When a player is infected they will eventually turn into a zombie and are informed by the game but the other players are not. The player can choose to be noble and kill themselves or they can wait and play as a zombie in which they have to kill all the human players.
If the game has chosen voodoo zombies then any voodoo related characters will have an added bonus in combat which will make it beneficial for the other players to team up with them. However, at the start the game may choose one of these characters to be a villain. The villain will be responsible for the spread of the zombies and will have different objectives such as killing all the players and stopping them developing the story. If story related characters turn into zombies they will also have added zombie powers.
The characters will also have different attributes (speed, health) as well as specific skills (driving, voodoo powers). The character will increase in attributes by killing zombies and saving people but the skills will level up with the progression of the characters story. As the story is progressed this this will change the character so a soldier will be promoted from private through to major. The character's zombies will also level up but will be independent of how far the human character has developed. This would lead to some players specifically turning at the start and trying to level up their zombies.
For the game to work the city should be fairly persistent so if a car is burnt out when a player returns later it will still be burnt out. The spread of the zombies should be modelled so there would be an outbreak point and the zombies would spread through the finite population (infinite spawning enemies are annoying). The furthest locations from the players could be very roughly modelled but as players get closer this should change so that every human and zombie are mapped and exhibit simple behaviour. In the near vicinity of the players everything would be modelled and more complex behaviours exhibited.
The game could be a very simple top down shooter such as the early GTAs but would be better as a TPS (Recent GTA Games), FPS (Left 4 Dead) or even switching between the two. The key aspects of the game are cooperative play with the risk of betrayal by the villain, multiple story arcs and a modelled city. This could easily be developed for other themes such as werewolves, disasters such as floods, organised crime, etc.