No Diablo 4, but a new Necromancer class and a recreation of Diablo 1 in Diablo 3 instead.
The Necromancer will be added to the game in 2017 via a paid character pack, so not as part of an expansion. The Necromancer abilities weren't detailed in the opening ceremony but is inspired by the Necromancer from Diablo 2. The Necromancer is playable at BlizzCon this weekend.
Command a new army of the dead. The Rise of the Necromancer pack is coming to Diablo III in 2017. pic.twitter.com/7R2Qj455S1— Diablo (Season 20 is LIVE) (@Diablo) November 4, 2016
Diablo 1 will be recreated in Diablo 3 courtesy of an anniversary patch that's arriving on the Public Test Realm next week. The patch, Darkening of Tristram, will take you back into the cathedral and down into 16 levels of monster-filled dungeons that will include the four main bosses from the first DIablo game.
There will be a special "Glorious Retrovision" graphics filter to make the game look pixelated and grainy, and character movement will be locked to eight directions.
Blizzard also said it would be added two new zones for free to Diablo 3: Reaper of Souls next year. They are the Shrouded Moors, and the Temple of the First Born. They'll be accessible via Adventure mode.