I love the idea of a server wide event with stages and quests, scenarios, and just a ton of build-up for the expansion, but I really don't think it's going to happen. I think it should happen, and it would be totally amazing but the main reason I'm pessimistic is because Blizzard doesn't like to develop a whole lot of content with such a short lifespan.
Remember that for Cataclysm, there was the Elemental Unrest and for Pandaria, it was the Theramore's Fall scenario, which was later re-used for MoP end-game. I just think looking at the past should remind us that we really have to temper our expectations for this sort of thing. I'm not saying there won't be an event. I'm just saying I think it's probably going to be something really simple.