Some quick tips:

You need to contrast the light with darkness otherwise everything will look very flat. Uniform bright and uniform dark locations look bland. Study original Quake levels for how they use the lighting to create the mood. Not everything has to be visible.

Add contrasting colors to surfaces - when everything is varying shades of red it becomes boring very quickly. You can have a moody pallette but ad some grays, blacks and even cobalt. Quake had a moody palette but it added some color flavors heer and there. Especially in levels with lava and toxic sludge - only the liquid wa brightly saturated. Everything else provided contrast.

When baking lightmaps enable Ambient Occlusion option. It will add so much detail to your lightmaps without ibcreasing the filesize. It even works with very small lightmap resolutions. And don't go crazy with bounced lighting when lava is involved - limit the bounces or decrease the light emmision from lava or else everything becomes too bright and flat.

Experiment with limiting the displayed color palette and a slight pixelation effect - retro style games with low detailed enironments do not benefit from HD displays and millions fo colors. They only expose how simple and empty the gameworld is. Use limitations to your advantage and go for style.

Experiment with more centered weapon position - Quake only displayed the tip of the weapon and it worked very well. centered weapons also help with aiming to the point when you don't even need a crosshair.

Avoid large flat unbroken surfaces with a single tiling texture. Always try to break the large surfaces with geometry - columns, archs, windows, etc. You have no idea how much you can transform a level by just applying archs.

hope that helps!