Caveat:  I am not remotely qualified from a technical standpoint to give you any sort of critique, so take all of my thoughts with a grain of salt.  :)

First and foremost, I think you could call this song finished as-is.  It's really cool and atmospheric, and it definitely evokes an environment in my mind.

That being said, if you want to expand on it, one thing I tend to try when I feel like I'm stuck on a particular song and I want to add more  to it is a change in mood.  For instance, at the moment, you have a rhythm in the background that pervades the whole song.  You might consider adding to the song by segueing that into something faster and more intense, and then back to the original calm mood for the conclusion.  The reason I'm saying this is that your song has me picturing an early level of some kind of Metroid-ish sci-fi side scroller, where it's more about exploring and less about action.  But then, I'd imagine the action might start to ramp up a bit as you get further and further in, and then you might go into a room where the mood is completely different and suddenly you're fighting for your life.  This is just a thought -- I realize that you may want to keep the song completely chill the whole time, so if that's the case you can discount what I just said.

Another random thought...  you might want to have another instrument, something with a low-fi sound (bit crunched or whatever the word is), quiet and filtered in such a way that it sounds like it's coming from a room far away.  You could have it fade in and out and be playing a melody that doesn't quite fit with the rest of the song.  It might be interesting and vaguely creepy.

Anyway, just some random thoughts. :)

Bart