There's several possibilities here, and I'm not sure which one is accurate. One thing though, is that all we know about next week is the announcement is Diablo related. It could be as simple as revealing the PS4/potential Xbox 1 versions. They may be holding the expansion till Blizzcon, they may not. I don't have the info.
Also, I don't think it's entirely right to rule out a quick WoW teaser or even formal announcement of the expansion at Gamescon either. It's really possible they could announce both expansions in succession.
Blizzard has a lot of cleaning up to do after the Diablo 3 launch fiasco and a large, globally watched stage could be the perfect opportunity for them to try an win back a lot of the fans they lost during the past year. Announcing the D3 expansion with a lot of "we listened and fixed this stuff" would be key to that. Considering that many of the producers and devs have already stated that always-on and Ah/RMAH was a mistake, but it's not as simple a solution as flipping the switch immediately, an Expansion would be the perfect opportunity for them to correct the issues, including adding an offline mode and total removal of the RMAH. Considering most of the people quit/boycotted Blizzard after launch and would most likely not pay any attention whatsoever to Blizzcon then Gamescon is the perfect place to make the Diablo name appositive thing again. Thats why we could see the expansion, but then again it's Blizzard who hasn't exactly been smart recently so who knows.
I'm still certain we will also see something WoW related next week too. If Blizzard is serious and wants to keep to releasing expansions faster and since the common assumption is March/April 2014 for that to be a reality, I can realistically see a trailer and some small tidbits with a "please look forward to Blizzcon" wrapping it up then an announcement during a panel at Blizzcon ending with an "oh by the way we're opening up beta servers next week." considering the subscriber dive in the past year I think Blizzard may use the advantage of a bigger audience watching Gamescon while also teasing more info, if for no other reason than to sell more virtual ticket passes. This could be an attempt to bring back people who have lost faith in WoW and could motivate them to actually pay attention to November.
As for "The Dark Below" I'm leaning toward WoW as I would like to see something different and new in D3 over a repeat trip to hell, I'd actually like to see a crazed Imperius as the main villain over Adria or hopefully not a "look Diablo's returned....again!" storyline. I know the writing is about at the level of a Disney Channel cartoon, but I'm Hoping they make things a bit more interesting this go around.