I have this idea, and NO I'm not trying to take over KingsIsle, I'm gona add some things to give them some advantage.
Those who have paying accounts, and wizards who are level 35+ (Or whatever half the level cap will be in the future!) can download an application that lets them design they're own world. Here's how:
You'd walk around as a little Wooden Construct thingy, and you could put down things.
-Monsters You'd control their attacks, HP, all stats and everything!
-NPCs You'd put them donw, and control their dialog and quest stuff.
-Quests You'd make forward and side quests. And design their rewards!
-Rooms, areas, and dungeons Make new areas and zones from defaults.
-Collision Points These may be tricky, but you'll have to label what's wall and not!
-Other Any other things you'd need!
All this would save to a file, like a RAR file. Then another thing, to test it.
I'm not sure, it seems a bit complex. It wouldn't exactly work the way you planned, unless there as lots of preprogramming. I could see people buying/winning things for their private realms like monsters, bosses, etc, but it would involve complicated programming if you want to label whats solid and isn't. Besides, the skilled crafter or crowns player can link their houses into a private spiral with teleporters!