You can up the darkness and depth without making the background fully black by adding shadows and increasing contrast.

Wow surt! That looks amazing! Thanks so much for the edit! Seeing your shading and how you use NES colors inspires me to stick strictly to NES limits. I really need to rework some stuff.
So about the shadows... I actually did add shadows to a previous iteration. I removed them because it was going to be very time consuming painting in correct shadows everywhere through the entire game. I really wanted to create a few tiling backgrounds and use those throughout the game. I definitely prefer how shadows give the scene more attention to detail though. I think I might be able to split the difference...somehow. I can create a background with straight, simple ceiling shadows and paint stuff like shadows from extended tiles on another in-engine pixel layer. Still might be time consuming.
I mean, it looks more like you recolored Metroid tiles than actually did your own art. I see the Metroid doors and from there I'm not sure what's yours and what isn't...
Really? I think it looks fairly different from recolored Metroid tiles. Both scenes.

All art is mine, made in PS and PyxelEdit. It is Sci-fi Metroid inspired though. 100%. And yah, I pixeled "Metroid style" doors. I like the way they transition from room to room. They also work in-game exactly like Metroid doors too. Shoot and there's an opening animation.