Just a thought that popped up into my head. If anybody has ever played Pokemon Ruby, Sapphire and Emerald it's basically that: a secret base somewhere in the open world where you can decorate to your liking and invite others to see/hang out in it.
In the case of it being instanced, just make it so an unlimited amount of people can use the same spot and that the hideout they pop up into is completely separate from the open world.
Thoughts?
Edit:
Here's a link to a person's video showing off their secret base in Pokemon Emerald to get a good visual example of what I'm talking about: https://m.youtube.com/watch?v=5rlkx7x7OBk