Just kind of hard to decide what you would do in each realm. I could see Apocrypha being more puzzle-like in design, we all know what the shivering isles and deadlands are like. The Quagmire would be interesting, but hard to do. It could be a maze that changes forms every so often.
The Ashpit... even if levitation isn't in the game, they would have to introduce it temporarily. That or have the player use an item that makes invisible walkways form.
I could have fun in Boethiah's Attribution's Share. Garden mazes and impossibly twisting towers.
Peryite's Realm (which I had thought was the Quagmire) would be nice, first hand look into pestilence.
Molag Bal would also be interesting, although perhaps I'm just sadistic. Probably won't happen since, well, he...dominates and censoreds people.
Sanguine would be nice, but...ESRB rating again.
Sanguine would be hard, its made up of 100,000 parts.