Blizzard currently has a public job listing for what it describes as a "first-person" game. Although details beyond that are broad reaching, it does mention that the role will work on the engine for what is a multiplatform game.
The post states:
When you consider that Blizzard recently released Overwatch, the amount of options become finite. A single-player adventure is unlikely given the company's attraction to multiplayer games. Thus, something co-op like Borderlands seems much more probable. It could even be an MMOFPS, although the genre has never gotten off the ground and has serious implications of risk.
The focus on engine development points toward the game being very early in development. Given the long development cycle of its previous projects, it is unlikely that we'll be playing this game within the next two or even three years.
It has been rumored that Blizzard is also working on Diablo 4, although its absence from BlizzCon 2016 has cast doubt on the existence of the project.
