@turpedo : Thanks! You're right ^^. Hopefully I can remedy the diffuse look, I did some hacky roughness map instead of the useless specular. I tried darkening the walls and changing the lighting so there's a bit more contrast. Huge fan of your work by the way! @Orb : Omg, the master himself! I added some color variation…
That's the thing, you don't need to know what it is in the background, it just looks cool. You could even do it with a plane mapped with a diffuse and emissive texture, you don't necessarily have to model anything :) I also added a slight gradient to add a hint of red at the bottom half of the image.
Looks like a lot of fun! I think you could do with a little more color variation in your diffuse textures. Most of that blue tint should come from your lighting and overall scene atmosphere, not your texture. Also the skulls should be a little meaner/scarier looking in general.