Diablo Jr. 
After the release of Diablo 2, a small number of Blizzard employees started work on a handheld Diablo spin-off game that was known as Diablo Jr Diablo IV Boosting.  since kids were its target audience instead of adults. Inspired by the Pokemon video games, Diablo Jr. would've seen multiple versions of it released, and would've been developed for either the original Game Boy or the Game Boy Advance. Blizzard played with making the game turn-based like Pokemon as well, which would've been a huge difference from the other Diablo games. Ultimately, the decision was made to cancel Diablo Jr. as it didn't fall in line with the company's vision for its products, and there would've been added manufacturing costs thanks to it being a cartridge game and not a PC release.

Diablo 2: Salvation
Blizzard registered a trademark for a project called Diablo 2: Salvation that never actually saw the light of day, but the general consensus seems to be that it would've been an expansion pack for Diablo 2. According to the Diablo Wiki, Diablo 2: Salvation would've had an increased focus on multiplayer, with guildhalls that players could work together to improve and upgrade. Diablo 2's playable classes would've grown with the proposed expansion as well, adding two more classes to the mix. Ultimately, the decision was made to leave Diablo 2 behind and start work on Diablo 3, though it's unclear if those proposed ideas would've been part of the Diablo 2: Salvation trademark or if that trademark was for a completely different project.

Diablo 3 (Blizzard North Version) 
When one considers how successful the original Diablo 2 was, it's somewhat surprising that it took Blizzard so long to release Diablo 3. However, it's not like Blizzard wasn't working on Diablo 3. As a matter of fact, Blizzard North was working on a version of Diablo 3 for years before that particular studio was closed down and development restarted to become the Diablo 3 that actually hit store shelves buy Diablo 4 Boosting.