Don't assume that Blizzard is wanting to reveal the next expansion because they want to release it early...
Their subscription numbers are likely TERRIBLY low at the moment, and they need to build hype. For them, hype = subscriptions and monies. I'm just assuming, anyway...
What do y'all think? Seems kinda odd they don't want to wait another quarterly to reveal at Blizzcon, if only to build hype.