I'm on that somewhat Outbreak side.
While I'd still like an idea of co-op here's what I'm thinking.
How about a game where you and friends HAVE to work together to survive.
Here's the catch almost everything must be done with the help of at least one other person. But the game splits you up multiple times. It can vary from splitting up to proceed or while it will take longer, to stick to each others assess.
Another thing is voice chat. You have to communicate but the farther you get the harder it is to hear them.
I'd like the Evil Within setting of the villiage. Or a hospital really.
One last thing could be that while guns aren't rare but the ammo is.