From what I can see your game consists of rectangular metal rooms with barrels in them. Thats the problem.
A level is a design process, you need to put some work in it. Don't hesitate to look at what other games did, real life photographic reference, artworks etc. Don't hesitate to learn the basics of modeling.
So far there is really nothing in your game that looks worth trying out to me. Its a bunch of corridors and bought media put into it. I dont mind you having fun playing around with FPSC, but since you want something with a little more personality and since you present it on IndieDB, I hope you dont mind my honesty.
Another good idea would be to play around with light color nuances. Never use plain white light. Its a general rule in lightmapping. Not only in FPSC but in general.
Custom HUDs and sounds are something you should worry about once your game reaches completion.
Quote: "I'm going to be releasing a demo for my game soon and i'm looking for tips on how to make an fpsc game feel more unique.
"
I'd say its a little early for that. Judging by your IndieDB profile.
Quote: "There is no story, you're just here to kill everything you see and blow up every barrel you see."
If this really is your entire concept, why not go for a cartoon/ over the top style game. Because this concept with military gear and grey to gunmetal grey colors doesnt really work for me.
-Wolf