As Xinh mentioned, the game has been out for 6+ years now. Except for when Blizz makes something entirely new possible through a patch (new features/new API), most great ideas have been thought of already. It's not often that you come across something totally new and groundbreaking. The
Pick of the Week here searches for those groundbreaking, creative addons, but much of the time it's just new, cool stuff (maybe with a different flavor or perspective to it).
I would say that the golden age of automobiles has passed, too. We are just expanding, refining, and reskinning them. If something new is added to a car that is groundbreaking, it's because of technology advancements (ex. new features/API in WoW) or a high level of creativity and ability to think outside of the box.