Many good suggestions itt. Player/guild housing is a feature that has been discussed for years and is, according to many different fan sites and forums and polls, one of the most anticipated features to be added.
Imo, there are so many good ideas i can't pick one :d but if i had to i'm going to say maybe Hinterlands?
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Thing is, this isnt something new idea from the OP. Its been discussed for years, and then never implemented. Even blueposts have said they love the idea but it would be a hard thing to do. Also, Rift did it with player dimensions and, i dont play Rift but i have friends that do, and they say that it is probably the best thing to happen to that game.
If Blizz were to add player housing, it would be as part of a new expansion definitely.