EDIT: Editing to clear some things up people seemed to frequently overlook/not fully understand the intent of, and also just taking in account feedback. Also to get everything in one post, since two lovely posters informed me that editing a post gets past the text limit.
Keeping a house should require a small fee for house upkeep/maintenance, similar to repairing your armor, that is paid monthly. Changed references to this as 'rent' to 'upkeep' to follow that line of thought.
The amount paid per month shouldn't be too high, realistically, it should be extremely negligible for any active player.
For some example prices:
Small = 250,000 gil
Med = 500,000 gil
Large = 1mil gil
These are more examples for comparing the cost of one size of house to another, the numbers themselves should not be taken as what would actually be put into the system, or even anything close.
Anything in your currency tab should be able to be used as a means to pay the upkeep. With the vast amount of currencies we have in the game, many of which become completely obsolete and unused, while still being awarded to us very regularly, we should be able to use these currencies as a means to buy tokens to pay for the house upkeep.
Allow people to pay in advance. This would be very helpful when it comes to real life emergencies. Let's say you have to stop playing because you have to switch jobs, and money is tight. By paying the upkeep for your house ahead of time, you could cement your hold on your plot for up to 6 months if you needed to, while your sub goes inactive, and you tend to your real life issues. It would be a good way to help people that know they might not be able to play, or even afford the game for a short time, but allow them to keep the house they worked so hard to obtain. This could also cause issues with people dodging the auto-demo, but with the right checks and balances to limit the amount of time you can put a payment down for (or even increasing the price for every month you want to pay in advance), it could be a very helpful system, while not being game breaking.
Allow the option for people to open an upkeep donation box on their house, so that if a house is open to the public, and people enjoy using it for RP, gpose, or what-have-you, people can help that person maintain and keep that house.
I understand that some worry that this comes across like a punishment for people owning homes, but if the numbers are well balanced enough, an active player with a home shouldn't have any troubles paying. At most it would punish house flippers, people with a house on every character, and the people that are inactive in the game, but log in once a month at the behest of an email just to keep their house as a status symbol/trophy.
Until housing is no longer a finite resource, I feel like a system like this should be put in place
Adding: The argument has been made that people do not want to pay more after buying a house, but with a system like this, maybe the initial price of buying the house could be lower to counter balance. Keep in mind also, for the “buy it and done paying” mentality, would you then also argue that repairing your armor should be removed from the game? Once you buy that armor, why should we have to still pay anything else for it?