Island Sanctuaries might not be instanced housing, simmer your hopes

[GeminiReed]

[T]his game's server architecture does not seem well-suited to open-ended instancing, as opposed to "at most X number of things in use"; their workaround seems to be that anything that is truly open-ended in terms of number of active instances has to be a solo environment that gets run largely (save for interactions with specific systems) on the client-side, rather than allocating a server-instanced environment.

(edit)
When you say "server architecture" are you referring to the hardware or the processes that make up the collection of services that our clients interact with? If it is a physical datacenter issue that's a breathtaking level of incompetence I struggle to attribute to their operations team. IMO it has something to do with the either the design or implementation of the ward system that makes it require so much physical hardware.

If Island Sanctuary is a form of instanced housing, I strongly suspect it would be one where you could decorate and garden and play around... but not invite guests over as that would require server-instanced environments. And that seems technologically feasible within this game's environment... but probably not what folks would want from "instanced housing".

As Shougun remembered it was stated in one of the Live Letters that other players can visit.