We can always use environment concept art.
Right now, the game lacks a singleplayer mode, so we just have a (admittedly fairly disparate) selection of multiplayer maps. The multiplayer maps are disparate namely because they are made by different individuals (whose individuality, of course, shines through), which is good and bad, and because we do not yet have a library of modular components with which to build maps, which is entirely bad.
If you have anything in mind for what a complete map might look like, go ahead! That's a tall order, however, so if you have an idea of what a map might look like overall—for example, what a particular, random room in a research labratory might look like, which would provide something for a mapper to extrapolate from for the rest of a map—you could draw a few concepts for different areas. What would be really awesome (but not strictly speaking required) is if you could then draw detail views (i.e., isometric and/or front/side/top views) of different components of a room such as obviously reusable things like cabinets and tables, or less-obvious things like repeating wall and floor panels. Take a look at the tutorial I linked above and the components that the author broke their map down into to get an idea of what I am talking about.
I do totally understand if you have some sort of assignment to do and you'd like to focus on environments first, so you could just start with those.
Here's some generic ideas that haven't really been explored:
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Scientific research laboratory
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Some sort of futuristic hotel / casino / shopping center complex (think Rainbow Six: Vegas set in the future, I guess)
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A military base, perhaps with hangars full of spacecraft or whatever
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Any kind of map with a highly discernible geographic feature. Think of the dam level from the original Half-Life. (A base at the bottom of a gorge, for example?)