Well it’s pretty good work, considering I can’t see any seams or distortions!
Since the sky takes up so much of the composition, one thing you could do to make it more interesting is change the weather. I’m picturing a very severe almost surreal storm with abstract cloud structure and an interesting light source. Something like this volcanic lightning is what I’m picturing http://monscooch.files.wordpress.com/2008/05/volcano.jpg
If not for the weather, a massive ‘Independence Day’ inspired ship hovering/attacking comes to mind, or maybe some kind of abstract structure towering over the hills.