The first problem I can see with adding playing housing is it either has to be free and available to all players either from the start or once they reach a certain level, or it has to have no direct effect on game play. Allow me to explain. Blizzard's philosophy in game design is such that when they add something new and interesting to lengthen the game experience, but that is tangential to the main game (i.e. not a new dungeon, but a new feature), they don't want it to be mandatory. This isn't because of player complaining (which they do no matter what happens, it seems) but more because they don't want to make something that initially seems like a fun side project into a mandatory get for anyone who was serious about the game. For example, imagine that they introduced the player housing that cost 20,000g. Obviously, it would take some time (for most of us) to rack up that kind of money with the game's economy as it stands. If that house gave some sort of advantage, as I have seem suggested, for example anything you cooked there would be slightly more powerful, all of a sudden home ownership is mandatory by high-end raiding guilds. Therefore if the player house is going to be anything more than cosmetic or storage, it has to be available to everyone. For an example of this design philosophy in action, see achievements. While they offer rewards for completion (titles, mounts, non-combat pets, tabards) they don't give you an advantage one way or the other (the exception being for the uber achievements that give out a 310% speed flying mount like the Heroic Raid Meta and the Holiday Meta).
Personally, I'm in favor of Blizzard just giving everyone access to a hovel at level 1 and allowing us to level it as we level our character. Allow me to elaborate. Upon reaching their race's capital city, each character, by the grace of Thrall (Varian Wrynn? I don't play ally and could not be bothered to look this up. Haha!) each character is granted a small and rundown house. Very small. Very rundown. At this point, you never have to do anything to the house that you don't want to do. What the house looks like and how much it can store or what conveniences it will offer you, will be entirely up to you, but the main game altering feature I envision, trophies, will be available to all, no matter the size or look of the player's house.
One of the things Blizzard has said, if I remember correctly, is that they don't want to invalidate old material or older zones by making you skip them all completely. With the trophy system, I think we can reinvigorate the older zones, specifically older dungeons, without having to do anything as silly as a Heroic Mode for Wailing Caverns (I take it back. That would be awesome.) The way trophies would work is you have 5 slots in every player house again, no dependent on size. Every final boss from every dungeon in Vanilla, BC and WotLK drops a trophy, usually their head but sometimes their hide or claw. It would depend on the boss. This item would be lootable by all members of the party. When you got back to town, you would place the trophy in one of the 5 trophy slots and it would give your character a Pride buff. The Pride buff would vary based on the trophy. For instance, completing Mara would grant you a trophy that increased all stats by +2. Completing Ragefire Chasm would grant a trophy that adds +1% experience to all kills. Now your Pride buff gives +2 to all stats and +1% experience to all kills. This would work for up to 5 buffs at once rolled into the Pride buff. In this way, Blizzard could at once encourage players to not only experience old content, but experience it at the appropriate level. What person leveling from 10-80 would turn up their nose at a buff to xp? Obviously, the higher level the dungeon, the better the trophies buff all the way up to Arthas' Helm which will grant you your life back (just kidding, WoW still owns you). Now for the non-combat goodies.
In addition to the trophy aspect, players will want to be able to store things in their new abode. However, it's just an empty shack! The solution? A new secondary profession called Carpentry. Carpentry uses various ores (or maybe bars) to make nails and other parts as well as granting the ability to harvest lumber from various places around Azeroth (Warsong Pine or Un'goro Mahogany anyone?). Using these materials, the Carpenter can start to repair and build additions as well as furniture for his home. Create a closet that can hold sets of armor. Create trunks for holding potion and items. Create a work bench for your main profession. Make yourself a bed that slightly increases the rate at which you accumulate rest. The higher your carpentry skill, the larger and more intricate the furniture you can make. Also, at certain skill points similar to how enchanters have to make new rods every 100 skill points or so, Carpenters will have to expand their homes to create room for the larger furniture by gathering a diverse and large amount of level specific materials.
Now the player house will start out as being in just available from your capital city. However, a ways down the line, and for a fee, you will be able to install a teleporter in one room of your house. Having done so, you will be able to purchase access to portals in each capital city. In this way, you can set your Hearth to your house and exit into whatever city you wish, adding extra convenience.
The final idea I have is an expansion of the player housing scenario to include the rest of your guild. The Guild Hall. Each guild will have the ability to create a meeting place and hangout for their membership. It won't be cheap, though. It will take all of the guild's members working together and gathering the mats to finish this project, though once it is completed, there will be room for expansion. Guild carpenters will be able to craft lounge furniture and other decorative items that serve no real purpose other that posterity. The big deal though, would be the Guild Trophies. This system would work similarly to the Pride buff players can get from their own homes, but with the Guild Pride (which would read as "
This kind of thing is always fun to think about. I'm sure it will never happen, but I just wanted to throw my ideas out there.