I want them to open WoW up to custom player content, it is the cornerstone of basically every other Blizzard game.
Custom scenarios, custom dungeons and raids, custom battlegrounds, custom zones, all taking advantage of 3D assets and game mechanics already in-game. With even a watered down, simple scripting system the community would come up with some spectacular shit. In some way it is taking the best part of private servers and adding them legitimately into WoW.
Of course players want to use this system to create a custom house in Nagrand. Even better if you want your house to be guarded by raid bosses and patrolled by elite mobs for visitors to encounter. The possibilities would be endless.