See, the thing with housing is it's a way they can utilize a shitload of assets they've already got built and sitting in the game files. It's really just a matter of turning the countless doodads in the game into collection objects, populating the game world with them (via drops, treasures, quests, achievements), and giving you a way to display them in a house of your choice.
I imagine that it'll launch with a somewhat limited scope in comparison to the twenty years worth of assets they've got to pull from, and naturally they'll want to grow it and implement more things over time to keep players engaged with it.
But some things I can say with near certainty:
- Players will want to be able to choose where there house is, just having instanced apartments in Stormwind and Orgrimmar isn't going to cut it.
- Players will want to choose what type of house they have -- size, style, racial theme, etc.
- Players will want to be able to actually choose the placement of their decorations so that their space feels unique, I don't think just socketing trophies onto slots on the wall will be satisfying.
- Players probably won't be satisfied with just a single warband house. It really does need to be handled per-character, so your goblin can have their house in Undermine and your blood elf in Eversong Woods, your dwarf in Khaz Modan, your worgen in Grizzly Hills, etc.
Personally, the idea I've had in my head for a while now is a "Homestead Act" system. Horde and Alliance decide to reward every hero of Azeroth for their service by bestowing them a land-grant. This allows each character to claim a plot of land from one of various zones in the game and build a homestead there. There might be requirements to claiming these plots, like driving off monsters, completing certain quests, or reaching reputation/renown milestones.
After you've chosen your plot of land you can then build your house. They could probably have whole game systems built around this. Different styles of housing for different races and reputation factions... maybe even a whole secondary profession like Architecture.
They can then use phasing and Delve style instancing so that other players can visit your house, and maybe some sort of overhauled Hearthstone or key system to give you and maybe even other players quick access to it.
At the same time I think those systems could be fleshed out to also allow for a parallel Guild Housing feature, and maybe even a Warband Barracks feature or something like that.
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I mean they can deprecate it. Most players aren't using Void Storage across all their characters, but it's reserved storage. They can replace it with a reclamation vendor to recover items that are rare or no longer obtainable and give players a chance to migrate their items out of it before removing it entirely.