Mobile Diablo Immortal is only one of the two projects set in the rich Diablo universe that are currently under development at Blizzard. While Immortal was announced during the BlizzCon 2018 opening ceremony, and instantly became one of the biggest subjects of controversy of 2018, we know nothing officially yet regarding the second project that might very well turn out to be Diablo IV.
According to Kotaku's Jason Schreier, the first version of Diablo IV was scrapped in 2016. Codenamed Hades, the goal was to take the series in a different direction, taking inspiration from Dark Souls. "It would be a gothic, challenging dungeon crawler. Rather than maintain the isometric camera angle of the first three Diablo games, it would use an over-the-shoulder, third-person perspective."
The Dark Souls-inspired game was developed for two years, until 2016, when Blizzard cancelled the project. Schreier's source said that “it was not shaping up at all." The developers who used to make Hades later put together the critically acclaimed Diablo III DLC titled Rise of the Necromancer. The rest of them started working on a brand new version of Diablo IV, this time codenamed Fenris.
Blizzard’s has been working on this version of the game since 2016, and some who have seen it say they’re optimistic about the direction. The game is directed by Luis Barriga, the former lead game designer on World of Warcraft. His intend is to move away from anything that was considered cartoony and immature in Diablo III, heading into much darker and more gross direction similar to Diablo II.
Fenris is still in early development, and likely won’t be out until 2020. Blizzard is apparently trying to make Diablo more social and is planning to add some online features to Diablo IV. According to Schreier, the game's status is pretty advanced, but Blizzard has decided to not announce Diablo IV at this year's BlizzCon to avoid the Project Titan fate and that title's public and ultimate cancelation.