Depending on what style you want your game to be in, don't be afraid of using different techniques. When I want something rather simple but yet rather realistic - I take snapshots of what kind of scenario I want, then I edit a few major details in photoshop and end everything by putting on the Cutout filter.
ofc this only works for one certain look or two, but if you want your background to merely be just a background and nothing noticable. Be sure to blur out the contrasts. Add an overlay of a bright/dark color or something.
Though, I have no idea why I'm handing out some sort of lecture right now, I like your style and you should keep it up
You're way better than me at these kinds of backgrounds. It looks really good all in all ^^