I would love for my house be moved to its own instance server. The wards feel excruciatingly dead as they are, especially the subdivision wards. At least in an a house instance I know it is supposed to be just me and my friends. Plus it eliminates the chance of trolls during crafting or roleplay (yes, I am on Balmung)
I think people would be surprised with how often houses are used, even if you do not see it, people can be on during different timezones than you, log in just to check vends or craft real fast until expansions, or whatnot. I think the amount of truly inactive houses are very low.
As for mortgages- I paid my house in full in-game up front. There is no loan to be repaid nor is the house not paid off.
Other games are able to do dynamic wards were new wards are added as-needed (Or automatically give a house to all characters), short of a few server upgrades, I am not sure why SE cannot do this as well.