It is most likely the expansion for Diablo 3. Why you ask? 1. The main soulstone that contains all the lords of hell is the black soulstone. There is one more stuck inside that stone, Leah, who was tricked by Diablo himself. So my theory since I beat it is that you will be tasked in trying to save her and you will have to either 1 venture into the stone or 2 release her and possibly the lord of hell.
As hinted by one of the Blizzard Devs, this may be a redo of Blackthorne. Said dev said he was working on something cool, but couldn't directly say, but here's a hint and took a picture of Blizzard's timeline on when games were released and directly in the middle of the shot is Blackthorne.