Diablo IV, which just received its first expansion, Vessel of Hatredd, was definitely a success for Blizzard. Even those who don't love the game must admit that, at its core, Diablo IV sticks closely to the tenets that made the franchise a tentpole in the genre.
However, as revealed by Jason Schreier (via Wired) in his latest book 'Play Nice: The Rise, Fall, and Future of Blizzard Entertainment', the first iteration of Diablo IV was something very different from what we got. Following Reaper of Souls, the higher-ups decided there wouldn't be a proper second expansion for Diablo III, so Reaper of Souls director Josh Mosqueira started working with a small team on a whole new concept. The project, codenamed Hades, employed an over the shoulder camera with action combat inspired by Rocksteady's Batman: Arkham trilogy.
Furthermore, this concept even had roguelike elements such as permadeath. Whenever the player character died, the user would get some kind of perks to facilitate their next attempt. While intriguing, the idea quickly ran into some obvious issues. Batman: Arkham's signature action combat was designed for a single player game, while Diablo IV, like all the games since the very beginning, would need to support cooperative multiplayer. According to Schreier's retelling, it didn't take long for Mosqueira to realize that himself. He left Blizzard in July 2016, founding Bonfire Studios, which to this day appears to have made no games of its own.
Following Mosqueira's departure, Blizzard decided to scrap the project and go back to what made the franchise great in the first place. That obviously translated into a longer wait for fans, but given the Diablo IV we actually got, perhaps it is best that Hades did not materialize after all - such a radically different game might have killed the beloved franchise altogether.