Thinking about gameplay lately I noticed, how much higher and lower levels improve the fun in a map. One of the very few maps which go towards using both horizontal and vertical gameplay is nmp9, which is one of the oldest maps.
Therefore I'm just gonna throw the idea in here: How about a CTF map with vertical gameplay? Would of course have to be a sci-fi map, I could imagine some duomo-like design. The basic idea would be both flags approximately next to each other quite at the bottom of the map. It must be arranged in a way that you can't reach one flag from the other, but instead have to go downwards to some teleporter-array bringing you back up on top of the map.
An advantage would be that there would be absolutely no need to make such a map symmetric, as the bases are next to each other. This would allow some interesting designs.
I hope I could make my idea clear, if not feel free to tell me. I don't know if my idea is worth looking into or even feasible. As I said, I'm just throwing this in here as I'm no mapper at all, but if anyone wants to try such a design, I'd be happy to think about it more detailed.
Edit: Very poor drawings added in a comment.