Everyone's asking for it (for a good reason) but here's my vision for it.
The concept borrows heavily from Apartments, in part to show how the framework for this already exists. Keep in mind this is a basic concept and isn't meant to address every single thing (biggest issue right now is how to deal with house sizes). This also isn't going to address FC houses, although I feel a lot of it applies to that as well. This talks about houses, as in house+yard, to clarify any misunderstandings.
The core concept mimics Apartments. There's a central hub with useful NPCs from which you can teleport to your house. All the relevant NPCs like Junkmonger, Material Supplier, Housing Merchants and the likes, including an FC chest and a Market Board will be available in this space. In this way we might even get more of a community feel than the current Wards enable by pure volume of players having to use this particular area. Apply instances as necessary.
Upon purchasing a plot of land, players will be able to preview the layout of the house as some houses have different yards and house location within the yard based on plot. While instanced houses will be free of actual location constraints, this will add flavor and keep all houses of the same size from being exactly the same.
OPTIONAL: there will be actual view of the ward your house is supposedly in. Houses can be generated at random completely, or selected at random from existing houses in your residential area. How confusing to have your neighbors change every time you load in!
"But what if I want to have people visit me?!"
Having a list of houses in a residential area is going to be horrendous, given that in this utopian setting everyone can have one so long as they can shell the gil for it. Therefore I suggest the action of "Invite to House". Upon accepting the invite, the player in question will have the option temporarily to enter the landlord's house. Instead of having an endless list of houses, each player will see the list of houses they have access to, be it that its their own, their FC's, a shared estate, or if they were invited over as guests.