First some ideas to reduce demand for housing
Personalized Airship Drydock
Purchase price: 200,000 gil per airship (max 4)
Location: Instance area accessed from the airship landing in each major city. Treat this similar to apartments where everyone will have permanent access to their drydock.
Purpose: Let individuals (and FCs) without a house create and dispatch airships without the use of a company workshop.
Residential Greenhouse Gardens
Purchase price: 100,000 gil per plot (max 3)
Location: Instance area accessed from somewhere in the residential ward/subdivision. Treat this similar to apartments where everyone will have permanent access to their greenhouse.
Purpose: Let individuals (and FCs) without a house use 8-patch plots to crossbreed seeds and grow plants.
Second, some ideas to increase supply.
Additional Wards
Instead of additional wards to existing cities, add housing to other cities such as Ishgard, Idyllshire and Mor Dhona. These should not be limited to FC only. Those that want the neighborhood feel of wards can pay the full price for it.
Instance housing
Purchase price: 50% of the ward price. Instance housing is unlimited, there doesn't need to be a high price tag for it. Also offering a lower price creates an incentive for people to relocate and free up ward space for those that "must have" the neighborhood feel.
Demolition time: None
Price reduction over time: None
Upgradeability: Allow plots to be upgraded from small to medium to large. The cost of the upgrade should be the difference in price between the original size and the upgrade size.
Relocation: Allow people to relocate from Wards to Instances and vice versa.
Third, managing flipping.
You can't stop flipping with any reasonable measure but you can reduce the market by reducing demand for housing and making it a legitimate and monitored system instead of forcing people to make unenforceable agreements through reddit or other places.
Player sold housing.
Allow us to create a deed for the house then place the deed on the MB. Once the deed is created the house is locked from use until destroyed or used. Players can list the deed on the MB for whatever price they want with the knowledge that they can't use the house while it is up for sale. If they chose to cancel the sale then they can destroy the deed themselves. Creation of a deed will not halt the demolition timer. This sale would also allow all furnishings to be sold with the house as a package deal.
Housing transfers.
Allow players to transfer housing ownership from one person to another. This would be similar to the trade system but with the key component of limiting how much a house could be sold for to its maximum plot price. Price can be set to 0 to make a gift to a friend. This transfer would also allow all furnishings to be transferred with the house.
If I wanted to "fix" housing, all of these ideas are what I'd start with. I know they aren't likely to happen and I'm sure there are a lot of server and software limitations that will make at least some of them impossible. But this thread isn't for possible or likely, just for ideals.
Please include your thoughts or ideas on how to improve these or post your own suggestions.