Hmm. I'm not sure exactly what the point of the artwork is. Is it something to give to whoever may mod the game and build this building, or is it meant to be turned into a more finished peice of concept art? If it's meant to just be something for a modder then I suppose as long as it contains all of your ideas it's okay, although still I'd think about pushing it a bit more, perhaps indicating that the interior of the buidling is more than just a black hole. Also, what's the thing on top that isn't colored in at all? is it meant to be included on the building or is it something that needs to be erased?
If you want to take this and make it a bit more finished, I'd determine a light source, then render the building and ground around it with that light source in mind, as well as adding indications of an interior. I'd also add some buildings around it and make it a whole scene.