There was that great, HUGE map that had the Dominions magic "planes" in the sides. You didn't have to teleport there IIRC, but it was so big that you wanted to. Besides, going to the Astral land would pay off rather quickly.
I don't remember the name, but you might have seen it. I suggest playing it, deciding what you want to do better, and starting in small scale. Three-layered map with underground tunnels and cloud-cities floating above the lands and connected with bridges sounds simple enough.
There's also Hollow World, Dom:PPP map with separate underground and paradise areas.
" This is an experimental map with three levels. The Primal Plane is magically rich and well defended. There is only one entrance to the Primal Plane. The Underworld is also quite well defended. There are several entrances into the underworld, including underwater ones. The sea provinces also have two levels."