Here's an alternative idea. i don't know how well it will work, but it might be worth giving a shot:
Build some prefabs. These don't have to be overly complex, just build things you know you'll use. I've got a spawn point that I'm very happy with. I've got it set aside for use later. I know the dimensions of it so I can plan off that. Don't do it for everything, just basic elements you know you'll use repeatedly. Like sections of corridor made to fit together and with 'end caps' that have doors on them. That gives you some elements to worth with.
Next, pick a shape. I don't mean a brush, I mean a shape. A star, a triangle, a square, an S, a +, an octagon, an 8, whatever. Draw that. Just a fairly regular shape. This is the flow chart. And by flow chart I mean this is a sketch of the paths of travel.
Look at the shape and figure out where the important spots should be. All of the map types boil down to points. Capture points. Move object from point A to point B. Keep the other team away from your point. Look at your flow diagram and think about where these ought to be. This will likely depend upon the game type you're working on. Single points (KotH) are centered. Attack/Defend maps tend to put the last point in front of the Defenders' spawn and then space out the rest with a gap between the first and the Attackers' spawn. CTF and Granary style CP maps space things out equidistantly with the intel or final points for each side located near spawns.
You now have your flow, your important points, your game type, and a good idea of how they interact. Now divide it up into parts- look at each point as its own section, and the areas between points as sections. Think about what you want to do with each one.
Points are best set up so that they can be accessed from multiple directions, and defended from multiple locations, and each counters the other. A Heavy who stands here is can cover there, but he'll be vulnerable from [/i]over here[/i]. Try to make sure that there's simply no way to cover everything at the same time with two or three people. Make sure that the main flow remains unobstructed, and you can get from one point to the next without having to do a lot of doubling back. Gravel pit's a good example of this because the ridge in the area for A leads from BLU spawn up to A, then arcs around and brings you down to the tunnel to B, which encourages you to make a U-turn into the building to hit the point, and then keep going straight to the entrance to C.
The areas between points are going to be similarly important. Bottlenecks are good, particularly right before you get to a section containing a point. Multiple pathways are great, especially ones tailored for specific classes, like Spies and Scouts. If there's a bottleneck there'll be fighting. If it's wide open, people are likely to retreat or push forward quickly. It's difficult to stop a rush across open ground, just because there's so many angles to cover. Short paths should be dangerous, long paths should be safe(r) but boring. Keep your direct routes well lit, let the side paths stay dim. The more direct the route, the more likely it should be big enough to accommodate a whole team running through it.
You should now have a 'map' that consists of a big path sectioned off with some dots on it, and smaller paths indicating side routes. Think about it in three-dimensions. You can make these side routes go horizontal, sure- but you can also go vertical. Change things up, both in terms of altitude but also design- nothing too fancy just yet, but we're getting to the details so it's good to start thinking about this. Is this part in a building? Alright- then maybe the main route is the ground floor, an upper side route is among rafters, wires, and HVAC ductwork, and an underground route has utility boxes, pipes, and metal flooring. Make sure each path is distinct enough to be visually interesting, but keep a theme going.
Right, so now you've got all your general bits, you've got your ideas for themes, your paths, it's time to start putting it together more distinctly. You should have ideas for props you want to use by now, along with textures. Start checking out props and see what their dimensions are, make notes for each one. Try to make sure it all blends. Keep the red versus blue idea in mind, and know who's home turf each area is (in Attack/Defend it's often all RED territory, in balanced maps you usually have distinct RED and BLU areas with middle ground being more neutral). Come up with a solidified theme for the whole map.
Now start applying the prop size requirements to the areas they'll be used in. This door is 186 HU high? Then the room will have to be more than that to make it work. The wall supports are 216 HU per section, with a 32 HU gap between supports? This path's length will need to be able to be broken down into those numbers. And so on.
And now you're basically ready. Put together a map that uses basic dev brushes and prefabs that matches your final design dimensions. Start running alpha tests. See what works. Isolate problems to specific areas. Try reworking those sections or paths. Once the basic problems are hammered out and it's decently playable, start swapping in those props and textures you picked out. Work on a section at a time. Get it looking good, test it regularly and use the feedback as fuel to keep you working.
Pick out a couple areas where you might do easter egg things, like the control room in 2Fort or the bomb assembly facility in Gold Rush. Build those separately with only one or two guidelines to keep track of- basically things like 'this will be visible through a window here, in these dimensions.' Have fun with it, attach it to your map, and go from there.
Then it's just a matter of things like cubemaps, sound environments, and tweaking your item placement.
(And now that you've read all that, no, I've never done the above, nor have I ever finished a map; I'm mostly extrapolating from what I've read and just coming up with ideas off the top of my head. But it sounds good, right?)