I think, this being a community of mappers, if anyone has a cool idea they'll likely just make it themselves. If you're stuck with some ideas for maps, check out valve maps, and check out some of the tutorials on the site:
We're not going to give you ideas on what you can do. The first part of making a map is a combination of simply pondering and drawing.
Last time you were driving, you saw this really cool rock formation. Maybe you should try making a map that focuses on that rock formation. You watched a video on this statue found underneath the sand in Egypt. Make an Egypt-themed map with that statue in the center next to the point. Etc., etc.
So you've gotten an idea in your head of what the map's all about (There should be a reason for why people are fighting over the area). Now go grab some graph paper, or any paper really, and draw. Sketch a top-down view of each section of your map, write some notes, and if you have to, sketch a 3D version of some areas to better understand what you want to accomplish.
Move on to Hammer. Start making brushes that go with the basic geometry of your design on paper. No need to detail yet - you want to make sure it's fun to play before you spend time detailing it. When it's done, add an _a1 to the end of the map name (cp_name_a1) and compile it. Check Interlopers for any errors in the compile log, and go in-game and test out your map. http://forums.tf2maps.net/showthread.php?t=14430 Use that as a checklist to make sure it's ready to test. Then post the map on these forums, and get it tested in a gameday.