The key issue is real estate. Where would Blizzard put guild halls so that the world didn’t get filled up with guild halls. While there are numerous other issues of concern, such as the economics of storing additional items in a guild bank/strong box instead of the bank, real estate is really the most serious issue.
One thought that I had was to have multiple distributed instances (like instance dungeons). Rather than just one (potentially crowded) instanced gateway perhaps Blizzard could use some of those vacant buildings in each of their cities and sell access to one as a guild hall. Say your guild officers find a nice place in the Dwarven Quarter of Stormwind and have 500 gold to buy a guild instance there.
I don’t even begin to know what would be in a guild hall (swimming pools and masseuses) but at least it would provide one more thing high level characters to spend money on. It might be nice to have a dragon kill point scoreboard or a stat board for Raids and loot. It might be nice if there was an in game message board for wish lists and help requests.
What do you think?