As has been mentioned before, Ishgard would be a great place to start instanced housing or multi-sized apartments. It's not a large place - the city itself is built much like the Goblet, a "town" on top of a tall island-like spit of land. There's nowhere to build a sprawling area for housing. So I think this would be a good place to experiment, especially if the housing could be nearly unlimited, given that data isn't being given over to "always-active" open-world wards.