>>1017539
Hand-paint surface details. Add layers of dust, rust, and scratches. Bake in lighting and ambient occlusion since these aren't PBR. The knife in particular is awful, and you'd be better off photocompositing bits of real knives on top of it.
Also, your textures are too high-res for how low-poly the models are. They're bordering on looking like the HD texture pack cat meme. Cut them in half at least.
If you're trying to do the pixelated software-rendered look you really should use a 256 color palette that encompasses the entirety of the game. Aside from being more authentic, it will also add a good bit of contrast that you're sorely missing. Look at Quake, Hexen 2, Chasm the Rift, and other early-3D games for examples.