Whoa, I was actually wondering what the effect might be on the Ceiling/Walls if the fog was removed XD That was super fast! I do think the fog effect works wonderfully, especially with the floors and ceilings. I'll give it a go!
The old tool I use also has the ability to add a "Backdrop" image at the far end of the screen, rather than just a ceiling/floor; which worked well with any dungeon/maze designs that did not have a ceiling (Think open air hedge maze or a Town/Village), a Sky or Cloud panorama or some such. This might not work right in your tool, but it's the only other thing I can think of off the top of my head that might be neat for somebody!
the Skybox works great and is an excellent addition and something that I will use in one of my own games.
I have added a new Texture button to the right window. It loads the default texture and the skybox is hidden by default.
I will hold off on uploading a new file until I have done a little more testing and updated all the help files for all the newly added content. Expect a new upload in a day or two ;)