Suspense. In the same way as something like Ju-on is scarier than a more gory, in-your-face action-packed style thing, unsettling scenarios are scarier than waves of zombies. I have to say I also really liked the music in Luigi's Mansion - obviously it's more of a family game, but I liked the way that in certain areas, how unsettled he was was shown by the way he whistled in a quavery fashion. Obviously I'm not saying to do that again, but something along those lines, where the character wasn't all gung-ho, and there was some sort of visible tell of how scared they were.
I remember playing one of the resident evil games way back when I were just a primary-school nipper - I think it was Nemesis - and there was a bit where you're running round a building and he's trying to get in or something (if I remember right). Again, the building of tension is a good tool.
Obviously this is just going down one avenue, and wouldn't necessarily apply to whatever route you guys want to go down, but some sort of sense of vulnerability would be good. Maybe even a couple of situations where the best course of action would be to avoid conflict and try and sneak past. If it was a zombie game, for example, maybe an area with many zombies in so that it would be physically impossible to kill them all. Could also be fun if you'd saved recently to unload on them with a chain gun and see how many you could kill before you died - you know what I mean? A lot of people I know enjoy doing that sort of thing as much as actually progressing through the campaign.
I think there are a few things which would be most important:
- Appearance - it wouldn't have to be commercial quality obviously, but some more muted colours, darker shades and some fog-style effects for outside could set the mood well.
- Music - vital for setting the tone. Quiet, suspenseful, maybe with something frantic for conflict which would be a shock should you run round the corner suddenly into an enemy. Scales and modes like Harmonic Minor, Phrygian and Locrian would probably suit this.
-Plot - there needs to be a genuinely interesting plot. I wouldn't play a game if the only aim was to run around killing zombies constantly. A decent storyline would create interest, and possibly heighten suspense as you'd feel more involved with the character.
Also one more afterthought - nope never mind I've lost it. That's plenty many suggestions for now though. Hope some of them are useful.
"A West Texas girl, just like me"
-Bush