Personal and Guild housing
Ultima Online had an awesome system. You could build pre-build houses, but you could also build walls, stairs, doors, etc. separately and place them so you could make custom houses, castles, and even entire villages.
Also, Lineage II had guild houses I believe.
The former would probably be difficult to implement in this game. But Guild Houses I think wouldn’t be too hard since they would be instanced. I think this would be cool to have and might even be a further deterrent to server transfers since you would lose your Guild House. Allowing us to customize it would probably be the hardest part.
It’s already been confirmed that guild housing and personal housing is on the way, and they’ll be released in the same patch.