In a series of Tweets, renowned developer David Brevik talked about the canceled expansion for Diablo 2, as well as the original idea for the sequel.
IN A NUTSHELL:
It looks like the iconic hack'n'slash game Diablo 2 could have received a second expansion (the first one was Lord of Destruction, which was released in June 2001 - exactly a year after the release of the base game). Former Blizzard North studio employee David Brevik shared information on Twitter about the project, which ultimately never made it beyond the concept phase.
All indications are that the expansion in question would have added the cleric class to the game, but it's worth noting that the idea for this character was most likely a bit different from how it's typically presented (the character probably wouldn't be a specialist in healing and supporting allies).

Information about the second expansion for Diablo 2 also appeared in Stay Awhile and Listen: Book 2 by David Craddock (via Eurogamer). He mentions, among others, one of the ideas - a base for a guild called Stieg Stone (in tribute to the developer Stieg Hedlund), the members of which could help the player progress for money.
The expansion was being created 6-12 months before David Brevik left Blizzard North. This was one of the reasons why it was never finished, although the main reason was the fact that the company simply had a lot on its mind at the time - patches for Diablo 2, an unannounced project, and the early version of Diablo 3 which also never saw the light of day.
Speaking of Diablo 3, in David Brevik's recent tweets there appeared a thread related to this game. Interestingly, according to the first plans of the developers, it was supposed to look noticeably different from what was finally released.
Some fans pointed out that the idea for this expansion seems similar to what Blizzard is planning for Diablo 4. David Brevik referred to it in this way.
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