I've been thinking a lot about Update 13, and CoT's plan for instancing all city districts on a massive shared world. In addition to its effect on open world PvP, this scheme also adds some interesting options for story-telling.
As an example, suppose they add a set of special, Halloween-themed instanced missions at the beginning of October. Then, two-weeks later, all characters which have completed those missions can switch to instances of the city which are being invaded by ghosts, goblins, etc.
As another option, suppose you have a set of missions which has two possible endings, depending on the actions of the player: EndingA and EndingB. Characters who have completed the missions and gotten EndingA can then enter special city-wide instances which resulted from EndingA, while characters who made other choices in the mission set can enter the city-wide instances for EndingB.