This is here for reference purposes. This article is really not as good as the rest, but I’ve kept it up because there are 1 or 2 good points to be picked up.

Inhabitants
In your sci fi maps you must give consideration to the inhabitants. Unless you are designing your own monsters you will have to make your levels fit with the existing ones from the game. The monsters from Quake2, for example, are very militaristic and this is reflected in the layout and textures (lots of brown and grey textures, concrete structures, sharp corners etc).

Lighting
Though coloured lighting is more likely to work in sci fi maps don’t get suckered into overdoing it. The rules still apply. Flashy lighting will not work. Think Blade Runner rather than Doctor Who.

Texturing
In a future setting metal and techie textures are generally more fitting than brick and stone, which is more for a modern setting. However that is just a general assumption, stone/concrete structures can also have a futuristic feel (like Quake 2 again) if used in large structures. The idea is that people will have seen concrete before for instance, which creates the familiarity, but they won’t have seen it in such large constructions, which creates the futuristic aspect.

Map Structure
When making your maps think industrial, organic, technological. Use piping, have circuits and exposed framework. In the future buildings and other structures are patched together and built on top of the old.

Don’t think that making something weird or strange will make it a good sci fi map. The Xen levels from Half Life were generally unpopular because they were so detached from reality. Things should still be recognizable. Unreal might have been set on an alien planet but there were places like mines and temples.