I think basically just a more customizable version of the pso2 personal quarters would work fine. Basically make the room more interactive for people that come in (like maybe incorporate some version of the practice mode so people can fight a stationary target or something), and have more item placement slots and like rooms you can change the shape of instead of having to use the furniture walls. And more customizable permissions for who can and can't enter the personal quarters.
I think the furniture system would also be good to incorporate into alliance quarters if they ever add them too, where the guild leader can also decide who can change the furniture. Otherwise for the guild quarters I think a similar system to Warframe that also has teleporters into alliance quarters, but also have teleporters that could be used to enter members personal quarters if they have them set to allow alliance members in.