Blizzard will host its own "Reveal Ceremony" presentation at Gamescom next week.
The main event will be livestreamed on Wednesday, 23rd August, at 5pm UK time.
A preview video will tease its impending announcements on Monday, 21st August, at 5pm as well.
As for what Blizzard is announcing, the company confirmed that a new animated Overwatch short film will be shown during the Wednesday presentation.
Any other announcements are all speculative, but it seems like the studio is due to announce a new World of Warcraft expansion soon. Looking at its history, Blizzard announced the MMO's last expansion, Legion, during Gamescom 2015, so there is a precedent for this sort of thing.