Some interesting pictures that I found on the web :
The Cyberpunk city
Alien ice world
Mass Effect Noveria
This concept art is from "Star Wars: The Old Republic". The thought of having a terraformed environment in a frozen world seems great.
Some kind of asteroid or moon
Floating Mountains (avatar style) in an arid landscape.
Another futuristic landscape, though I doubt it fits with the Alien franchise. But looks great anyway.
Call me crazy, but I like the idea of seeing a city on the ocean, or under the water, why not? (Ahto City, planet Manaan from "Star Wars: Knights of the Old Republic")
This seems like a crossover between Alien and The Thing
The decaying vegetation can also become "Gigerseque" or Xeno-like
Ilos from Mass Effect has a similar landscape