The composition is just fine, you need more details and elements though. Personally i won't tilt the horizon line that much if my subject is the background (when your viewer have to turn his head or the image to know what's going on then you're going over the dynamic aspect).
About the technique itself i suggest to stick with round brush or chalk brush cause if you abuse some non solid brush your image will become all fuzzy. As i can see you already know about value separation but try to use it more, maybe following the 3step rule (light grey background, medium grey midground and dark grey foreground), generally teaches you how to put the camera inside the image, it's something really valuable.
If there's something else i can help you with let me know
Dude, please do not claim the title of Newbie. You are clearly way beyond that. Us actual Newbies will get depressed...
I dont think I’d mind having any of these on the wall. An art director would perhaps ask for more detailed concepts. (Seeing as the job of a concept artist is to define how something will look). But this style is perfect for i.e. book illustration.
Keep in mind the difference between Design and illustration. (and then theres abstract art, but we all know that doesn’t count).