And when I say player housing I'm talking about more than just an instanced location that you can decorate and invite your friends to. I think there should be a range of types of houses, ranging from a small apartment to a large castle. I think all players should start with the apartment and have to work to upgrade to a better house.
Collectibles would come into play here. There would be crafting stations for like cooking and alchemy that would allow one to make stat buffing food or potions using collectibles. For instance combining a vial of pure water with a sweet whitecap might create a light healing potion. With food maybe a feast could be crafted and everyone who comes and eats some of the food gets an hour long buff.
When a player owns a castle he could hire a group of adventures. They would start at level 1 and the player would control how they level up or set it to auto, and the player would upgrade their equipment. These adventurers could be sent into dungeons and bring back loot they found there, in addition to earning xp. That is if they succeed in completing the dungeon. Sending a level 1 team into a level 20 quest would result in a failed mission and no loot. I'm thinking like you can send your group into the pit and you might get a muckdoom when they come back, if you're lucky.
One last thing I'd like to bring up is relics. Relics are items that give bonuses, either permanent or temporary, to your toon when they're displayed in your home. Like statues or a mirror or a painting. They could be acquired through questing or raiding or maybe some can only be found with your group of adventurers. Furthermore, different combinations of relics displayed at once could give a set bonus. Bigger houses would give more spots for relics.