Instead of guild halls, we instead got Guild Airships? Airship shipyards could be added to Stormwind and Orgrimmar. Guildmembers could go to the shipyard and donate gold and mats to build an airship, with bigger airships requiring more mats (and thus larger guilds). When completed, each type of airship class/model would be customizeable. The guild airships could then be flown into the depths, and guilds could fight each other over control for underground refineries and such for their faction like in Guns of Icarus. There would also be player housing involved, where players could go below deck and seemlessly enter their own phased rooms and decorate them inside.