So, ok, when I think Survival horror I mainly think zombies. 3 types of zombies to be exact.
So, after a series of events involving a End of the World prophecy that went unfulfilled, groups of people who believed and prepared for the end of days vanished. There were three groups: The first group, up to around 14,000 people committed mass suicide the day of the event. Some did it in groups, others were found dead in their homes. Some even went on killing sprees before taking themselves out, bringing the numbers of casualties up to 18,000. A second group, somewhere around 110,000, went into hiding somewhere never to be heard or seen of again. The third group, only about 500,000 or so, were said to have believed that the world was going to end and embraced it. Once the day came and passed however, a majority of the believes said they were over it. Though reports of strange activity were on the rise.
Fast Forward, 15 years in to the future. A terrorist group known as "The Survivors" emerge, claiming to be the only humans to survive the End of Days and believed to have gone mad upon discovering the world survived and has thrived in the years they spend cooped up. They ran out of food and resources in their bunker, and slowly started to lose themselves to violence just to survive and learning it was all for naught didn't help their mental health. They vow to end the world themselves because it was supposed to happen 15 years prior. Cue their biggest target, a Bio-Lab devoted to human research and health where, unknown to them, a secret project developed to sedate the minds of people to quell mass panic in events such as the end of days was still in testing.
Then, BOOM the complex explodes.
First, the shockwave careens across the landscape, obliterating a nearby suburb and ravaging the nearby cities. A wave of untested chemicals sweeps across the ruins.
Now the chemical has two possible effects. For some it rests their minds to the point of no return, separating themselves from their bodies. They can still see, hear, taste, and smell, but they can no longer respond to it. Instead their bodies are running on their own by their infected minds, sedate unless provoked. Acting independently from themselves, lurching in a limp, relaxed fashion until provoked by fast movement, sound, and unfamiliarity. They can only watch as their bodies are literally turning into Zombies in every sense of the word.
The other effect of the chemical drives the infected individual mad. In other words they are in a state of perpetual provoked, attacking anything and everything for no reason what so ever.
Also, may cause mutations.
So now the world i going to hell and very few people can stop it. The governments of the world have sent military and scientific groups to ground zero, the bio-lab, to research a cure or method of fixing the mes the world has gotten itself into. Not as easy as it sounds however, as "The Survivors" have taken this outbreak as the cataclysm they were promised and have interfered in government evacuations.
This is were you come in, you are one of the few who are immune and the government needs you to help neutralize the outbreak. They have sent a team to come get you, but on the return trip you are attacked by "The Survivors." Now, stranded and on foot, you have to make it from city to city to reach the Bio-Lab. You must make it through vast, sandbox cities; teeming with zombies and extremists. Ammo and bases are spread threw out, but one shot could send a mob of infected in your direction.
There are several ways threw the cities:
1) Run and Gun your way threw the hordes, don't think twice about the fact that the thoughts and feeling of who they once were can still see you... unless of course they're eyes are gone.
2) Sneak. Alleys are good, but they are small areas and you could easily be closed in. The streets are stuffed with Zombies, so you can ether make your way around or you can push a button and begin to act like a zombie in order to not draw any attention. Nothing like stumbling at a briskly slow pace through an ocean in infected knowing one misstep could alert them. But that's the fastest way out of the city.
3) Vehicles. They can't take a relentless beating from running over zombies and they attract a lot of attention, but they are quick and usually you'll need one to make it to the next city.
Would also work great online, I'd wager. The more the merrier, there is more than one immune.