I mean what could they possibly announce that would justify a whole Blizzcon?
They already announced the D4 expansion, OW2 and Hearthstone aren't getting any significant updates.
OW2 losing PVE and Hearthstone actually lost 2 modes recently as well (Mercenaries is dead and Duels got completely removed)
Not a lot of exciting news to share from Blizzard I'd imagine.