I like the idea you have, thought I'd do a quick edit. One thing led to another, and then the quick edit got out of hand...
I'm no super artist, but here are a few things I was thinking about:Color
I think the colors you chose are pretty solid. The largest change I did here was reduce the color count from 25 down to 12. I also played with the hues and added some contrast, but nothing major.Form
I think this is the area where you stand to improve the most. As minnow said, there should be some gradation of value. Think of where the light would strike, and where it will not reach. Because you shaded all your rocks the same way, your piece looks very flat. Even though it is pixel art, think of the object in 3D.Composition
I really dig the idea you have here, reminds me of the desolation caused by the Nothing in the Neverending Story. Most of what I did here was remove/fix things that didn't make sense, and add areas of interest and places for your eyes to relax.
Hopefully this is helpful. Keep at it!