there are hundreds of ideas involving both game play and design have explored but have not released in any full maps. I'm posting these here instead of the new thread because most of these don't concern game play but design instead.
The Lava puzzle at the end of ZerTM_ht was a simplified version of an idea I once showed AquiRe involving a maze with a lava floor where you had to find sigils of protection and buttons that would raise a walkway around the maze. It is in one of the Mortisville maps, but too frustrating in its current implementation.
Other ideas --
One I demonstrated a while back, the idea of using a min light of 250 in the worldspawn entity and then using the light entities as deductive and additive brushes:
http://mortisville.quakedev.com/quake12.jpg
You do get a better since of control with the lighting with this method than standard lighting.
Another way of doing this would be to once again set min light to around 250 and paint on the lighting for each texture in its position in the level. Reuse of textures becomes severely limited, of course, and it is very time consuming.
Also, much more fun than the last method, is to separate out each room or area in a map, create five to ten thousand brushes worth of detail adding material textures without defining details, then go around the individually built maps and take screen shots that are the bases of the textures that you eventually use for the actual level. If you are careful with the lighting, this method can look extremely good.
Similar to this, I like to model in programs like Wings3d and Blender and take ray cast based shots of the models and use those as the basis of textures, like this one:
http://mortisville.quakedev.com/lamp3_5.jpg