I really love this piece. Great textures, great layout, love the detailing, and the lack of too many high saturation colors. I'm not sure what can be done in the program you are using but the biggest problem I see is the rubble piles and the interaction between those, the cracks, and your background. I'd personally like the background a bit darker, its lighter than a lot of your flooring so it makes it look like a floor above the map rather than fill around it, and this only gets worse near the cracks. Though there are a lot of options for the rubble, as you've mentioned, one thing that would help for me is a simple gradient on it so it gets darker as it moves away from the lighted halls into the cracks. Not sure if that's possible with your program at all.