TL;DR Instanced housing with the ability to upgrade your house would be amazing!
Instanced housing could be AMAZING! The idea for it is that it would be attached to the current areas and/or any major cities in an expansion (Sharlayan housing!?). An NPC at the entrance or in a special little alcove/building that lets you ride a boat, or airship travel to it. Boat for cities where there's water, and airship for the others. It could be an island outside the current housing area. Since it would be instanced, this would work for both personal and FC housing. The idea here is that it would work similar to apartments. And since it's instanced why couldn't one person have say, one house in every city? Wouldn't that be amazing? Think of all the decorating! (I believe SWTOR does this and I think it is quite cool, but theirs is more of a condo/penthouse situation. Upgradeable apartments would also be really cool now that I think about it.)
This idea is something we came up with to combat the current exclusivity of the housing. It would give the people who have WAY too much gil, more than they can spend, something to spend it all on! Furniture would sell a lot again and I think that would revitalize the crafting community!
Another bonus to this idea is that if you have neighbors who have decorating tastes that kind of clash with yours, you wouldn't have to look at anyone else's house! AND you wouldn't have to worry about how other people feel about YOUR decorating style! Isn't that nice?
As for the housing itself and plot sizes, I think it would be really cool if you could buy a small, and then upgrade to medium, and then upgrade again to large. Upgrading your instanced house would be a lot easier than moving in my opinion. But why stop there, what about MEGA MANSIONS?! The twin's parents house looks so huge and awesome, I would totally love to have a house like that! Think of the awesome parties!
We could also have way bigger yards with this idea! A lot of yards are cramped because people want a lot of big decorations, but even the large plots don't have much yard space for some of the stuff we can get.
Instanced housing could be SO good! And if someone wants to work up an exorbitant amount of gil and get an instanced house in every city AND fully decorate them all, why not? That would make for so much more gameplay! Especially if there was quests or tasks involved with upgrading the instanced house.
Anyway these are just ideas! Going through the plots today to try yet again to get an FC house in Ishgard, the wards are MOSTLY empty. Like 90% empty. Everyone just wants a large, now they're bidding on some mediums but I see a ton of smalls and mediums with 0 participants. This is an actual problem, and I have known people who have quit the game over not being able to get a large plot. Instanced housing would solve SO many problems!