I realize that this game isn't survival horror, but there is certainly a kind of tension felt when you're making your way through the desolate environment, especially when you're playing for the first few times (and don't really know what all is out there).
Pretty soon though, it becomes apparent that there are only a couple things to actually be afraid of to any real degree. And you're apparently safe when indoors from any real threats, which takes the edge off of looting indoors. I remember being in the hydro dam for the first time and always wondering if there was something else in there with me or not. And alas, there never was. Completely empty.
I think it would be incredibly awesome if there was just a bit more to be afraid of (indoors and outdoors). A way to do this that wouldn't necessarily destroy the overall realistic theme of the game could be a sanity meter (perhaps just the sleep meter). For example, when your sleep meter passes a certain threshold (and thus your sanity), you begin to have auditory hallucinations. And then at a certain point, visual.
There are so many possibilities with this kind of system. Furthermore, what about putting some kind of legendary creature in the game (like sasquatch) which might have a very small chance of appearing? Just knowing it's possible would keep you on your toes.
Just a couple ideas.