The unofficial information indicates that the first expansion for Diablo 4 will be titled Lord of Hatred, where players will have to face Mephisto.

Data miners revealed Blizzard plans for Diablo 4
Data miners have once again uncovered intriguing details that might reveal the plans of game developers. The most recent “victim” of their actions happens to be Blizzard and its Diablo 4. In the 2.0 version files (this version is currently in the closed test stage), there seem to be hints that the first big DLC will be named “Lord of Hatred“. That clearly shows that the main evil that the players will have to face will be Mephisto. But that is not all! It was also revealed that the expansion will also offer a completely new class called “Spiritborn”. This new class is rumored to be utilizing the forces of nature during combat.
Diablo 4: Lord of Hatred DCL – new region, new class, AI companions and raids
Besides the “Spiritborn” class, players will have a chance to embark on a journey through an entirely new land called Kurast. Well, maybe not completely new because the old-school player that played Diablo 2 should be quite familiar with it as it was initially introduced in that game. “Lord of Hatred” will also allow players to hire a companion that will be controlled by AI, just like in Diablo 2, but with a new twist. These companions will also have skill trees and equipment.
The last feature mentioned in the leak is raids, which would most likely serve as endgame activities for advanced players at high levels. This can be Blizzard‘s answer to accusations that Diablo 4 offers little to no activities in the endgame stages.
We have to wait till November, when at the BlizzCon event Blizzard should officially announce Lord of Hatred DLC and give us some more information about it. We will update this post once we know more.