It's not Diablo 4, but Blizzard did bring new Diablo news to Blizzcon this year. On stage at Blizzcon 2016, Blizzard announced that the Necromancer would be coming to Diablo 3. Blizzard co-founder Frank Pearce noting that players had been asking for the Necro since Diablo 3 first launched in 2012.
Like their Diablo 2 counterpart, the Necromancer can control armies of the undead and explode corpses. The male Necromancer was shown in the teaser trailer and screenshots, but there will be a female version as well. A blog post covering the class shows off returning and new abilities like Corpse Explosion, Bone Spear, Siphon Blood, Blood Rush, and Summon Skeletons. There will be a Necromancer-specific scythe as a main hand weapon.
The class will as part of the Rise of the Necromancer pack, which also includes an in-game pet, two additional character slots, two additional stash tabs (on PC), a portrait frame, pennant, banner, and banner sigil for an undisclosed price. Players will have to have Diablo 3 and the Reaper of Souls expansion to access Rise of the Necromancer. Blizzard will provide more information closer to launch, which will be in 2017.
The first Diablo kicked off the franchise way back in December 31, 1996. In honor of the 20th anniversary, Blizzard will be bringing the original Diablo to Diablo 3: Reaper of Souls with a new content update called The Darkening of Tristam. The update will include a 16-level dungeon featuring the four main bosses of Diablo.
The Darkening of Tristam will lock player movement to eight directions, just like the original title. In addition, the update will include a graphical filter call "glorious retrovision", making the game look pixeled and grainy. The Darkening of Tristam will be available for all characters and classes, but players will have to have Diablo 3 and Reaper of Souls to join in the anniversary action. The content update will go live on the Public Test Realm next week.
The Rise of the Necromancer update is confirmed to be coming to PC, Xbox One, and PlayStation 4 as long as Reaper of Souls installed. (Comes packed-in automatically with the Xbox One and PlayStation 4 Ultimate Evil Edition releases.) The Darkening of Tristam update isn't confirmed, but I doubt Blizzard would leave console players out in the cold.