So let's break this down, because stating that I believe a completely new WoW expansion will be announced at Blizzcon is pretty out there, especially given the fact that they'll be in the midst of launching the current one they've been working on.
Blizzard is on record many, many, many times stating they want to get to faster releases, to delivering an expansion every year if possible, or at least faster than every two years. Tom Chilton of Blizzard was recently asked about why they haven't delivered on that promise:
The reality is that scaling up the number of people that we have, to work on multiple projects at once has slowed us down. Honestly, it should have not come as a surprise to us. We increased the size of the team by 50% and the majority of those people had never worked on World of Warcraft before or any other MMO, so it is really difficult for them to create content right away, without getting up to speed. So we ended up redoing a lot of the content that we were doing for Warlords to make sure that we would get it at the quality level that we would expect.Having been on a team that literally tripled in size overnight and being responsible for their education, I can tell you it's extremely unrealistic to expect the team to not be negatively impacted in terms of delivering content. A good rule of thumb I have from experience is that it will take 3 - 6 months before a software developer can be truly independently productive on an established team/product. Closer to three if you have area expertise and/or just a lot of experience in general, and about six if you're totally new to the area and need to catch up. On top of that, the experienced folks need to take a lot of time mentoring the new team members, so their productivity is negatively affected as well.
This isn't a new concept by any means. If you're in any sort of management, The Mythical Man-Month is, frankly, a must read. Specifically:
"Adding manpower to a late software project makes it later" -- Brooks' LawI don't care what software project you're working on, this maxim holds true time and time and time again. Blizzard paid the price of violating that maxim with a later release than they wanted. However, now that the workforce is trained and up to speed, I expect that they'll be able to increase their content delivery velocity.
So if Blizzard wants to have expansions out every year, with possibly only a couple of raid tiers an expansion (I think 6 months per tier is the perfect amount of time for a tier of 10 - 14 bosses), then it makes sense that Blizzcon is their opportunity to announce this. Blizzcon next year would be too late as it would be when such an imaginary expansion would launch, though given how much they like GamesCom, perhaps they could wait until next August, but that still feels way too late for such an announcement, so no, I think Blizzard would likely feel this is the perfect time and place if this were the case.
With the launch of Warlords of Draenor the week after Blizzcon 2014, most of the excitement for it will already be at a fever pitch. Given the kinds of folks likely to be at Blizzcon or watching announcements and like are probably already getting fatigued with the amount of WoD information we've been inundated with, I don't really think the announcement of a new expansion would undercut the currently upcoming expansion too much. Frankly, I don't think there's much information about Warlords left to even go over. Even the cinematics have all been revealed.
We shall see if my prediction is correct. It's definitely a risky prediction, but really where's the fun in the small, incremental predictions? But we shall see in about two months if I am correct!