If you are on the Discord, which I highly recommend you join, we have an entire channel dedicated to inspiration for maps. Thinking of a proper layout/theme takes time, but it's something that gets better as you progress through your mapping endeavors. For now, Take a look at TF2's stock themes: Deserts, Forests, Tundras, etc. If you have a certain biome you like, choose a 'sub-level' of that biome. For Example:
Desert:
Mines, Caverns, Caves, Canyons, Plains, Etc.
Once you have that, Take time to study and research(such as searching 'mines' on google images) what you want to create, and try to influence gameplay aspects that you see from other mappers. As the great composer Hadyn once said: "Good composers borrow, Great composers steal!" Don't be afraid to incorporate aspects of a valve map, or even another community map that you like, and making it into something you like. However, don't go too overboard with copying things, make it your own style! I'm excited to see what you can create, cryo. Keep me posted!
