Have an upside down room, or a room where everythingf is normal, like furniture n stuff, but rotated like 25 or 30 degrees so its really trippy. i would so play that!

If you do add in objects rotated to be on the walls / ceiling, make them prop_physics, and have them start off, so when you shoot them they fall ;), parent the tv to the table though, I think that would be interesting. Also try to have like floors that break and different things like that.

Darker and more rustic/worn down textures.

Zombies! a good little area would be a pitch black sewer with enough water to wade through, with zombies. not very creative, but it works!

shivers

Make a room the lighting is normal then the lights go off and come back on and there are creatures around you and then the lights go off and come back on and they are gone a door slams open!

Have a corridor with metal grate on one side and have the zombie model that hits the mesh. also LOTS of blood and have a medical area with guts and crap.

Make a room where everything is cut in half. Reference: http://www.youtube.com/watch?v=M9De8E38eis

How about instead of waking up in a cell you like come home from work to ifnd nobody home? i'm not trying to recreate the whole thing but consider it. Anyway, i think there should be rooms with dead bodies but every time you would be about to discover a dead body the lights go out and you wake up in front of the room.

After solving a puzzle something would jump out at you and then it would allow you to go into the dead body rooms allowing you further in the game.

General ideas: -Random sounds of glass breaking and metal clangs.

-Random sounds of whispering, and make some animated decals (e.G.: Text written in blood) that only flash up like every 20 seconds.

-Use a trigger_look on a dark wall with only one bit lit on it by some dark light(possibly red), and after looking at it for like 3 seconds, a lightning should struck and a picture of a little 5 year old girl pops up and you hear child laughter. That one is a classic.(You should also make her appear fully later in the game and turn into something more scary and eat you or something).

-You hear extremely annoying screeching-screaming like sounds and you have distorted wiew(possibly drunk effect, also try to slow walking speed down), random scary things flying at you. -Use a trigger_look on a window to have you see a ghost fly by the window followed by a very deep blow of wind.