One of the strengths of this game has always been the number, and quality of the doodads that exist inside NPC buildings. Especially starting with Pandaria, the interiors of houses and buildings have lots of cool things going on. Furniture, weapons, heirlooms, light sources, etc.
What if the new (plot device) and (side villain) of this, or a future expansion began to work mayhem on a microscopic level, exactly like the mana wyrm boss of the second kharazan?
I’m not suggesting a campaign level of quests, just a lot of world quest locations that cause you to shrink down and fight inside a now massive building interior. Not instances, but actually scaled down and taking place within the world itself, with some additional location geometry that allows a few elevation changes.
Once this is successfully tested, it could be expanded to old content.