Literally 1 expansion per year (every 12 months on the dot).
If they launch WOD in September-October (which is just my guesstimate), the only way they could follow up with an expansion 12 months afterwards is if they've already got the story down for it now, and are starting to work on a cinematic, zones, quests etc.
The reason for this is because I can't believe you can make an entire expansion by itself in one year if you really start working on it only past the currently launched expansion (ex, they only really start working on post-WOD expansion right after they launch WOD).
The people working at Blizzard are human beings, and I don't think their company, especially now that it's so large and probably controlled by a fair amount of bureaucracy, still allows its employees to work unpaid 24 hour shifts and sleeping at their desks overnight like they did when making Starcraft 1 for example. (read the story I'm referencing here: http://www.codeofhonor.com/blog/toug...d-to-starcraft )
And considering they're always telling us how they've been increasing the WoW team, and how they don't want to invest ressources in a major content patch for pandaland anymore, the only logical way I can assume they're distributing all that massive amount of manpower is by working on both WoD and post WoD expansion simultaneously. That would actually be a very believable explanation as to why they don't have time to make any more content for this expansion. It also fits with a description a developer made at Blizzcon about how the story team already has plans for 5-6 expansions ahead, and how everything they're doing for WoD fits in a larger scheme of things for expansions ahead.
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This thread doesn't really have a purpose other than to say, don't be so angry that Blizzard isn't keeping its 1 expansion per year promise at the moment, and don't be angry if they don't keep it past WoD either, because they're either lying amount the vast amount of manpower they say they've hired lately, or they've just bit on a bit more than they can chew with the "1 year 1 expansion" announcement.
Have a nice day, and lower your expectations for the moment .