Over the years, Blizzard has gradually added more and more to the game. A lot of the things they've added have been the most successful things from other, less popular MMOs. Things like transmog, follower companions, single-player story aspects, pet battles, toy collections, etc were done by other MMOs first, and then added to WoW once Blizzard realized they were good ideas.
The one thing that is missing is Player Housing. Now, before everyone jumps down my back about why this WOULDN'T work and how WoW already tried and failed with Garrisons, let's just take a step back and consider a few things:
1) Player Housing has been both incredibly successful and incredibly lucrative in several other MMOs.
A. It's been successful in many MMOs, including Elder Scrolls Online, Wildstar, SWTOR, Guild Wars, and more. So much so that while several of these games failed. The housing aspect continued to draw players and revenue for the game.
B. It makes a lot of money for very little development time. Most housing assets are the game already. All the furniture, pictures, decorations, and house models you see around the world are already in the game. Put some of these the cash shop, and players will pay for content that's already been created years ago. It's basically an easy source of money for the Blizzard Store.
2) It is content that would stay relevant over every new expansion of the game. One of people's biggest complaints are that content gets abandoned from one expansion to the next, like Garrisons, Artifact weapons, etc. Housing would be something that just gets added to. New decoration models would get added to the game, but the older ones would still be there. If anything the older content ones would become more rare and coveted.
3) It helps the economy. Players spend TONS of in-game cash on housing. I spent something like 200 million+ credits on my SWTOR house and still had plans to spent much, much more. WoW needs gold sinks. Right now, the only gold sinks are mounts, and not enough people care about these for them to be truly effective. Imagine something like a statue of Warchief Sylvanus that everyone wants to buy but costs 1 million gold from a vendor.
4) I can span all aspects of the game, from professions, to pvp, to pve, to the cash shop.
A. Housing could easily add a whole new profession to game. One that can make decorations or various styles of buildings. If not a whole new profession, the current professions could craft various decorations, some BoE to sell, and others exclusively BoP.
B. PvP honor levels could award various PvP themed decorations. Such as weapon plaques, mounted skulls, or npcs to place in your house.
Arenas could award a statue of their character in the Gladiator armor for that season.
C. PvE content could drop various decorations from raids. Exclusive, Mythic only decorations would give raiders more incentive to push content.
I could easily imagine a NPC of a Naga could drop from the Azshara raid, and could be placed in your house.
D. And of course some decorations would be exclusive to the Blizzard Store. They're purely cosmetic and don't over any in-game advantages.
E. I'm sure you can think of other areas something like this would naturally fit, like rep rewards, timed dungeon runs, and achievements.
Now let's consider some of the obvious cons people will no-doubt bring up:
1) It will depopulate the world if people have their own player space, especially if you put things like Auction Houses and Banks in them. - No, I don't believe this will be the case. Firstly, because the game has more incentives to go out into the world now than is has had since Vanilla. World quests, world bosses, war mode all draw people to the various zones around the world. Garrisons had AHs and Banks even without these things. Secondly, Blizzard obviously doesn't believe having AHs only in the cities is important anymore as they've added a mount with a mobile AH. Sure, it's 5 million gold, but just make the player housing AH cost a huge sum too.
2) They tried it already with Garrisons and it obviously failed. - No, Garrisons were not player housing. Yes, they were the closest thing WoW has come to giving players their own player-space, but it was not housing. It was not customizable with decorations (barely, if you count the monuments), it barely allowed you to control the lay-out. The only space it had to display your collections was if you built a stable, pet menagerie, or archaeology finds. Real player housing would be much better.
3) This is Warcraft not the Sims. - Yes, but the two are not mutually exclusive. You can very easily make a WAR themed house. SWTOR was a good example of this where you could basically make your own private Sith Temple if you wanted. Make your player space into a walled stronghold, a mystical mage tower, a gladiator fighting arena. This isn't JUST a space to do home decorating. It's a mini-game with hours of content.
Now that we've seen some of the pros and debunked some of the cons, let's look at what player housing could actually LOOK like:
1) It should have a VARIETY OF THEMES. - If you are picturing an instanced section of Stormwind or Orgimmar, then that's pretty boring. Imagine instead being able to choose to buy real estate from around the entire playable Warcraft world. A floating rock in Outland, a frozen mountain top in Northrend, an underwater home in Vashj'ir, a tropical island off the coast of STV, a spooky estate in Duskwood, a tranquil retreat in the Jade Forest, or an elemental fortress in the Firelands... the possibilities are endless.
How would it work? Simple. How did Garrisons work in Shadowmoon/Frostfire? Each housing could make use of Blizzard's sharding tech, so that when you approach the house which you've purchased, you are only sent to yours and not someone else's. If you haven't purchased that property, you would only see the "for sale" sign as you approach.
2) It should be SOCIAL. - Encourage players to visit each other's houses. Give players the ability to set their housing to private (invite only, or key lists) or public (anyone can visit it). The public ones might become mini-towns in their own right, especially if one player managed to get all the most-difficult-to-find perks in his house. (AH, Bank, Rep Emissaries, Exclusive vendors, etc)
3) It should be HIGHLY CUSTOMIZABLE. - Perhaps select some basic templates players have to build on, but from there let them build how they like. Placing statues, npcs, buildings, and various other decorations all over the place. SWTOR's system of "hooks" was a little limiting, but made it so there were no lore-breaking houses like you see in other, more free player housing systems. (no jump puzzles made out of random objects).
Just Google some of the best player houses from SWTOR, Wildstar, ESO and you'll see why they are so popular.