It is a really interesting image and I love the concept. Drawing looks nice too. In your value study, you are using only about 30% of the possible value range - this tends to kill your focal point and to make everything look a dull gray.
I have made a very rough adjustement (I am not on PS now, so I do not have all the nice tools that photoshop has!), just to see how the image look with a wider value range.
It would need a bit finer touch, but I think it explains what I mean...It is still not going all the way to black or to white (often not a good idea), but is using about 70-80% of the value range now.