Blizzard's original vision for Diablo 4 would have changed the franchise in significant ways, bringing the dungeon crawler to the realm of third-person gaming.

Jason Schreier's new book Play Nice: The Rise, Fall, and Future Of Blizzard Entertainment chronicles some of the most interesting and pivotal moments of the game-maker's history. The book also discusses many of the cancelled projects and games, including Blizzard's own unique take on Minecraft.
One such story touches upon what kind of major departure that Diablo 4 could have been for the series. Blizzard sources tell Schreier that at the tail-end of Diablo 3's popular Reaper of Souls expansion, then-Blizzard president Mike Morhaime had put top leads in charge of pre-planning the next big Diablo game.
Diablo IV, which was then known as Project Hades, was originally led by Josh Mosqueira. The director helped save Diablo III with the Reaper of Souls expansion, which saw widespread success on PC and later on consoles.
Project Hades was actually a third-person action game with brutal combat, somewhat similar to games like Rocksteady's Arkham Knight.
Another substantial change was that Hades would have permadeath, not unlike Diablo's challenging Hardcore mode. If a character died in Hades, they would be gone forever, making the game into a true dungeon-delving type of experience.

Hades would have co-op multiplayer, but exact details weren't outlined. The game was just in experimental phases and hadn't been fleshed out enough for any kind of demo or trial release for consumers or testers--it was just a prototype.
Even stil, we have to wonder what a game like Hades would have done for the Diablo franchise, and more importantly, if it would have been a success.
Meanwhile, Diablo IV has since become the most successful game in the franchise, generating $666 million in game sales revenue in 5 days. The game has gone on to make over $1 billion in earnings in little over a year.