One of the main things that horror does well is make innocent things seem scary. Creepy Child and Monster Clown are two examples off the top of my head. Horror also tends to be very dark and brooding. This obviously prays on our fear of the dark, as we don't know what the hell's in there. Now, what if somebody inverted this?
My base idea is that something's sweeping the land any whatever it "corrupts" becomes extremely beautiful and picturesque. Of course, there's nasty stuff hiding in this transformed landscape, (camo?) and you are trying to make things right again. Once the area is restored, it looks... realistic and dull. What I'm trying to get at here is that we need to make pretty things scary and make the player glad to be back in the brown, dull, and dark city streets. Thoughts?
Also, something really nasty happens if someone's in the area when it's transformed. Maybe he turns into a baddie, which you will probably kill if you want to stay alive. Also, you should not find this out until you find what's keeping the area this way and destroying it.