Traditionally, World of Warcraft expansions have always been announced at BlizzCon, but there's no rule that says this absolutely has to be the case. And for reasons I have outlined below, I'm thinking Blizzard might be ready to diverge from the pattern.
Previously we consistently had a BlizzCon within a year of the previous expansion. Generally it floats between about 8-11 months, but always within a year. However Mists of Pandaria was released on September 25th, 2012 while the next BlizzCon isn't until November 8th, well over 13 months later.
Combined Blizzard's stated desire to get expansions out faster, I find it curious that they would take so long to even announce expansion 5. Given past experience it should be announced no later then late August and possibly as early as May. While there may be some creative reasons for delaying the announcement (ie they don't want to reveal ending details of MoP), I don't think those can really account for such a big delay. Metzen doesn't run Blizzard and if he really wanted to, for example, keep the identity of the next Warchief under wraps, I'm sure he could dance around the subject as needed.
"So the Burning Legion will know about the whole Horde civil war by this point and will think the Horde isn't as strong as they used to be. But Warchief...Not Garrosh is going to give them a serious wake up call."
So what gives?
I'm thinking BlizzCon 2013 is not intended for expansion 5 at all and is instead intended for the reveal of Blizzard's currently unannounced MMO. It's been in the works for a good few years now and it might be the right time to make it public. The leaked product slate from a while back did indicate that Expansion 5 and the new MMO would be released in the same quarter, and while the timeline has undoubtedly shifted significantly since then it makes logical sense it would at least be announced within a year of expansion 5.
Since they wouldn't want to announce two different games so close to each other, expansion 5 will likely be announced at a different event sometime this summer. Come May, look for Blizzard making a big deal about their presence at a major gaming convention or announcing a press tour for unspecified Warcraft-related purposes. I'd be willing to bet that's where we'll see expansion 5 announced rather then at BlizzCon 2013.