Another cool theme, I'm gonna have a go on this one!
Two quick sketches, one exterior and one interior, don't know which way to go yet, but I'll probably do a few more of these before I decide.
Crits and comments are much appreciated
@ Patrick_P
Cool choice of ratio, definitely one I have to try sometime. Your sketch is a bit dark but that gives it a cool feeling! As JoseVega wrote, just make sure the contrast of the pillars corresponds to the distance from the camera

Besides that, I'm really looking forward to the updates!
@ AlexTLe
Its nice to see your process, and the idea is great! Looking forward for more! That pillar structure of yours, in my opinion, begs to dominate the picture a bit more. If you want my two cents I would probably try to take the camera angle a bit lower towards the road and thereby make the pillar seem taller. (hope it makes sense, and please do take my advice with a grain of salt, I don't want to mess up whats already good!)
@ Papaya_Pig
Thats a beasty looking cave you've got! I hope you keep some of the texture in there during your detailing

About your photoshop trouble, I don't know of a way to personally change the background color to a chosen color. However, (this works on my pc (mac)) when painting in photoshop, hitting the F key switches the background color between blue, grey and black. Hope it helps!