There's most certainly going to be a new game set in the Diablo universe. The question is whether it's an expansion or a full game.
Diablo and Warcraft developer Blizzard has posted a new job listing that makes it clear a new Diablo project is in the works.
The listing is for an art director position, but it does not name the game, only the franchise: Diablo.
The wording makes it sound like this project is very early, which rules it out from being an expansion to Diablo 3. Judging by how long Blizzard takes to develop games, we're probably looking at a 2018 release of what's likely an entire new game.
Thanks, Shack News.