I guess I'll start off.
Ambient sounds. I think that the level's atmosphere could be enhanced with careful and subtle use of ambient sounds here and there. Some suggestions:NATURE
-Jungle ambience for overgrown outdoor areas. Thiink bird calls, monkey screeches etc. BUT this may seem at odds with the demonic invasion where you would expect the local wildlife to have fled or been killed off.
-Wind howls and loops for higher and more exposed outdoor areas.
-Gentle water tinkling for pools of water. Rushing waterfall sounds etc.
-Cave ambience, tumbling soil and rocks
-Tree and undergrowth rustlingBASE
-Computer beeping, humming and working sounds. Might want to avoid the QuakeII ones as they have become a ZDoom cliche of sorts.
-Various humming and buzzing noise for lights, big screens, hologram projectors etc.
-Sludge slopping/bubbling noises for nukage flow areas
-Klaxon for first lab area with warning light? Might be annoying though.
-Various sound sequences for the doors and lifts. As nice as the default Doom ones are, we have heard them a million times. With special custom sounds for different types of doors they will feel more real and memorable.HELL
-Background growls, snorts, laughs etc.
-Slopping oozing noises for scrolling fleshy growths
-Various weird musical effects
-Lava bubbling and sizzling
-Medieval or fleshy door/lift sound sequences. Hearing hydraulic doors in pure Hell areas is a real mood killer.
-Tortured screams and howls of pain for torture areas
-Screams of damned souls
And to top it all off use of appropriate reverbs in key sections. Think echoing caves, clanging air vents, weird muffling in Hellish corridors. Of course the background music + ambient sounds + gameplay sound + reverb effect could result in a horrible cacophany if not done subtly!
For additonal props, I would suggest adding in tufts of grass, small ferns and bushes, scattering small rocks and boulders and adding hanging vines into the outdoor areas. Could maybe randomise the size of the trees but perhaps that's taking things too far.
For other effects I would suggest using Ghastly's lavaball spawner in magma areas. I'm tempted to suggest the Doom 3 blood decals too for the base interior, but the art style is maybe a bit too different. Doom's blood is notably bright and pixely.
Edit: Also might want to consider liquid splashes and blood colour fixes built into the wad.
Player sounds could be expanded too. Add falling, drowning and surfacing, poison caughing, Quakeguy lava-screeching etc.