May have been mentioned before, but why not let an FC once it gets allied to its Grand Company, and has built its first airship the option to purchase a floating island like the sky pirates. You could make them in small, medium and large flavors for maybe 1, 2, or 3 connected islands. FC could purchase addition options like a building to let FC members have access to apartments on it, and a building to maybe have the workshop there, and maybe a building that is a single large room for a fc meeting hall. Single island could start around the cost of a large plot and go up from there. Access for non FC members if you allow it to be open to others would be only from the FC plot and maybe needs an outside furnishing or attachment for home.