I'd prefer to be able to "buy" a house in ANY and ALL of the capitol cities, old (Dalaran Northrend ) and new ( Dalaran Legion ). Not really a place to go sit at the auction or whatever, but a place you can upgrade with side things like a "transmographer", but it would be cool to be able to put up trophies for Achievements you have. Like if you're a demon hunter and you have the "Let me hold onto those for you" you can place the warglaives up somewhere. Make it so we can change interior design ( Carpet, Furniture, ect ) Obviously this is something you'll put gold into / maybe even a "building profession" so you can go out and gather materials to make the items. It would be cool to have expansion specific rooms (Portals to these rooms - again have to purchase / gather material - be a MAGE? )
These rooms will pretty much be that characters collection of items that the character ( class ) can use from that expansion. Tier / PVP / Weapons sets ( as you collect them they will automatically get put on armor stands for that tier set ) So a priest wont have a bunch of two handed swords / Plate armor put up everywhere. Honestly this would be a cool way to be a completionist. Legion room for example if you have all 3 artifacts they will show up there. Then to show off you can have people "view" your house. You can set the options to just friends / invite only / anyone. So people can browse through other people's homes to see them without main owner having to be there if you choose.
This isn't something that should be an expansion themed thing to be trashed and forgotten in the next. This should be something that is fully apart of the game and able to move forward. As new expansions release they can release new Furniture's, painting, collectibles, additional rooms for the future expansions.
It doesn't have to be just like this but just some idea's of what they could do for player housing.