Let's recap for a moment here.
They have 500 demo stations (which is comically astronomical), announced their huge presence back in April in which they stated they have new content showing for five different games, have had six week-long contests fan contest to hype their Gamescom event, have a dedicated webpage for their Gamescom presence, and seem to have a hour long livestreamed press conference where they've invited everyone from major websites to small fansites.
Despite this, there seem to be people who aren't expecting a major announcement.
Blizzard has exclusively announced WoW expansions at Blizzcon probably because WoW is their biggest draw there. Outside of release dates, it's all been Blizzcon. Based on precedent alone you can with almost certainty say no WoW expansion will be announced. Reaper of Souls was announced at Gamescon but it's not WoW, it doesn't fill Blizzcon.
Hearthstone was a smaller project so the release place is understanding. Curse of Naxxaramas and Molten Core were announced at PAX as well. Gnomes vs Goblins was a Blizzcon announcement. Grand Tournament? It gets it own event.
I can see Starcraft II and Diablo 3 getting expansion announcements at other events now (and before) because they simply don't have a big enough following for a lot of people to care. Blizzard's RoS midnight launch may be the last big event for Diablo.
What this all means is Blizzard's current announcements (based on their previous big ones) seem to imply a Blizzcon announcement. They could pull a 180 and announce it at Gamescon but their history says no. I would put an expansion announcement lowest (besides F2P) for gamescon. I would not be surprised if they announced a 6.3. Honest to God, I would not be.
Regarding major announcements, Blizzard says every thing is major like an additional Sanctum, a shipyard, or updated character models.