I've mapped both ways where I drew out on paper and followed it pretty close and also just pure freelance. Now I sorta use a combo of the two. I will just rough sketch a very basic "flow" to my map with cp locations, then I freelance from there. That allows me to be creative but also withing a certain structure of what I want to do. What I find helps inspire me when I sorta get into a mental block is to just go play the game. Sometime I search for maps I don't play often or have never played and I check them out as I play to get ideas. Don't be afraid to just start making something because you are afraid of losing the work because you will not like it. Start with basics and if you like it, keep going. If you don't, just group it and move it aside, you might come back to it later and have an idea for it. I do that ALL the time. I also will go grab some models and throw them in there around the area I am working on and sometimes that can help you with what you want to do with brushes.
Also, if you have a certain theme you are trying to stick with, go google some images and stuff. For example, if you want a factory look for a certain part of your map, go google some factory images. Same with cliffs, and stuff.
