It's been a long time since I got to play with Photoshop. I like the style you chose in this picture. It bring to mind pictures out of national geographic, miles of horizon with a huge sky. This is a very cool piece. The huge thunderheads, and the purple/white tone all lend to a very ominous, force of nature feel. Leaving out a specific color for the surfaces gives more strength to the sky, as it kind of washes out the ground and shadows. I imagine a burning bright moon barely obscured behind low, still clouds. A few white beams cutting through to the ground. What I would suggest most of all is brushes, particularly with the clouds, and with the contrasting parts of the rock wall. I think very distinct textures would bring this piece to life, without sacrificing its caricature quality. I could see it with exaggerated texture, brightly contrasting light and shadow, and a powerful moon to give it a bit more punch. All in all, I would like to think of this as a great concept piece, waiting to be revisited. Very cool.
dA has a huge selection of brushes to look through, [link]