Alien-themed maps and mods fit really well with Doom's atmosphere. No surprise one of the earliest full conversion mods for the game was an Alien one. Modder Kontra Kommando is making a completely new one from scratch, and it already has some really good-looking sprites and textures. Kontra Kommando said:
Took me a few days to fully re-colour the blood. The map itself didn't take that long, but its far from finished; it will be 10x bigger.
It will be a e1m1, e2m1, e3m1, and e4m1 replacement. Each episode replaced for Ultimate Doom will feature this same large sprawling map, with different item and enemy layouts, as well as different starting positions. I will name each episode replacement Scenario A, - Scenario D.
Here's a small preview of the map. Reminds me a lot of Dark Forces' first couple of levels!