A handheld installment in the Diablo series was once considered by Blizzard North, the developer of the hugely successful PC action-RPG, according to an excerpt from David Craddock's upcoming book about the studio, Stay Awhile and Listen.
After Diablo 2 was released, the Blizzard North development team split in two, one of which began working on the Lord of Destruction expansion, and the other went into prototyping. The latter team proposed a number of projects the developer could work on, including a Game Boy Color or Game Boy Advance iteration of the franchise tentatively titled Diablo Junior.
"Diablo Junior was intended as a single-player-only prequel to the original game," Craddock wrote, in an excerpt posted on Shacknews. "Taking a page from Pokémon's book, the team wanted to release three cartridges, each packing a different hero in the warrior-rogue-sorcerer vein as well as items that players would have to trade for in order to collect. Heroes started in a unique town before heading into dungeons and wilderness zones.
"Diablo Junior was ultimately put out to pasture (absent of cow levels, I'm sure) because of the steep production costs associated with developing handheld games," Craddock added.
Craddock also mentioned that another scrapped project was a second expansion to Diablo 2, which would focus on expanding the game's multiplayer features. Read the whole excerpt on Shacknews for more info on the Diablos that might-have-been.
Update: A video which purportedly demonstrates the prototyped, handheld Diablo was uploaded to YouTube in 2008. You can check it out below.