You can get idea's from anything. Reference photo's, books, movies, real world...
Usually it helps not forcing yourself to get idea's. Something that also helps greatly is somewhat the opposite: brainstorming together with others.
Oh, and Valve maps aren't perfect. No single game is. Try to keep that in mind - you don't have to get things perfect immediatly. Failure isn't a bad thing, as long as you learn from it.
I mean, I failed countless times. I've got a load of half-finished maps that I'll never release because I can't get them finished anymore. but I learned from them: I now see I always got stuck because I never planned the layout beforehand, and that always became fatal later on. Now, I've done a layout first and the map plays fun, and the best thing is that I can now get detailing and that's what I like most!
So just try some things, just try to make a map that you think is fun. If people don't like it, ask them why not and ask for advice. You'll get better over time, it'll shape your idea for fun and helps you get better idea's.
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