Of course, any 3D engine programmer will tell you what sounds simple on paper is a completely different animal when you try to put it on the screen. That's because modern 3D engines do tons of pre-processing to figure out all of the lighting and to calculate which parts of the level players can see from wherever they're standing. The moment you decide that the level is going to be random every time the player fires it up ... well, you've got to come up with a whole new approach for rendering it onto the screen.