I was talking with a friend about the period at the end of Burning Crusade when the crates began spawning in major cities. He reminded me that neither of us had really paid much attention - his guild was in the process of breaking up over M'uru/Entropius and I was tanking for a guild working on clearing Black Temple at the time. We were busy, is what I'm saying. So busy, in fact, that one day we found ourselves running for our lives from an Ironforge that was completely infested with the walking dead. Other players were now zombies. The auctioneers were dead. It was all chaos and madness.
Now, for a lot of people, the zombie invasion was a load of fun. It was new and different, something you didn't see in game every day. Some of my guildies went over to Orgrimmar and joined forces with Horde players they knew (Norgannon was a smallish and incestuous server in those days, all the Alliance and Horde players seemed to know one another) to form roving gangs of undead, laying waste to all, Horde or Alliance. They had great fun.
For me it was a huge pain in the rear end.
I couldn't get anything done. I get why people liked it. I didn't. My guild was on the cusp of getting Illidan down and that was all I was focused on, and it irritated me then (and still irritates me now to think about it) to find myself completely unable to do what I played the game for - couldn't get consumables, half the raid team couldn't get out of a city before being plagued, and forget zoning into the raid itself with waves of walking corpses attacking you before you could fly past them. I hold this out as a caution for the rose colored glasses people get when they talk about the Zombie Infestation - as novel as it was in hindsight, at the time, it was seriously bogging down the game. There's a reason it ended abruptly.
I think that negative reaction by players like myself - people who play the game for specific reasons, potentially on limited time or with scheduling constraints - may have kept us from getting more content like the zombie outbreak since. And I think it's time to consider whether or not such perishable content - content that only exists for a limited period of time and is then gone - has in fact a role to play in the game today and in the future.
I was disheartened when Mists of Pandaria launched with a scenario instead of an actual event - as important as the bombing of Theramore was, it really didn't seem very thematically connected to the expansion, and I really wished for something more on the order of the Elemental Invasion that began Cataclysm - love it or hate it, it was a real series of world events that had content that only existed for the period of time that it was describing and it linked thematically to Deathwing's emergence. That's why I actually liked the Battlefield Barrens weekly quest, and the fact that it existed - because it was transitory. It was content that existed for exactly one patch, to tell a very specific story, and either you were there and you played the game and experienced that story, or you didn't and you won't.
The downside of perishable content - the fact that it leaves the game - is a real one. It's basically the same amount of design and implementation work for the development team, and it results in content that doesn't last as long and thus doesn't give as many people a chance to experience it. In a game like World of Warcraft, which has vast spaces full of content, it seems odd and even a little counter productive to put something in the game knowing it's coming out - I farmed up a set of the plate warrior gear from Battlefield Barrens purely for transmog knowing that I had only so many weeks to get it all done, and now several of my alts won't ever be able to collect that set. My shaman and DK ended up just plain not getting there in time. But I think that very temporary nature of the content is also a benefit to it - it helps enmesh the events in player base mythologizing. Frankly, looking back now on the zombie outbreak, while I stand by my opinion of it - it was annoying, kept me from playing the game as I wished, derailed a lot of people - looking at how the player base has taken and built upon the events makes me think there's value to be had.
A big pre-expansion Warlords of Draenor lead-in could go a long way towards getting players to do the heavy lifting for the development team. People embrace the myth of how awesome the zombie plague was over the reality of it because it's impossible to go back and play it again - all we have are stories and videos to preserve it, and so its greatness grows in the telling. Imagine a month of slowly growing dread as strange new forces make raids on Alliance and Horde towns, then vanish after opposition arrives? Then progress the experience - the raids get bigger, they hit hardened targets like Stormwind and Undercity, and soon we have waves of Iron Horde pouring through the Dark Portal and a Gronn Siege Tank world boss attacking cities, dropping Iron Horde troops off for players of all levels to engage with. Then allow the event to taper off, let the players feel like they won a battle and pushed the Iron Horde off their very doorsteps, and bam. In a year's time the legend will only have grown.
Would it disrupt things? To some extent. But as much as I found it annoying years ago, I now think that disruption is itself part of the mythmaking - players tell stories about events that affect their play, after all. In the end, a pre-expansion world event is necessary, in my opinion.