The original Diablo is being remade in Diablo III and the Necromancer will be released as DLC according to announcements made during BlizzCon 2016's keynote.
Diablo III held the prime, spotlight position in the BlizzCon 2016 keynote. Capping off the hour-long event, Blizzard celebrated the Diablo franchise's 20th anniversary by announcing a mixture of upcoming free and paid DLC for Diablo III designed to pay respect to the franchise's history. The first major reveal was that the original Diablo was being remade as free DLC and shortly after that Blizzard confirmed plans to release the Diablo II Necromancer class as premium DLC.
Remaking the original Diablo within Diablo III was likely the big announcement in celebration of the franchise's anniversary. Blizzard is calling it The Darkening of Tristram, which will be a month-long anniversary event that unlocks Diablo from within the existing world of the third game. Players will be able to travel through a portal to "experience Adria's memories" and play through the content Blizzard has updated. That includes a special visual filter to make Diablo III look like its predecessor. The Darkening of Tristram will launch in January and it will reappear as Blizzard celebrates Diablo's anniversary in the future.
The second big reveal, and perhaps the most exciting announcement regarding Diablo III, is the unveiling of the Necromancer class. Blizzard is planning on releasing the Necromancer as premium DLC in a "character pack" that will include stash tabs, character slots, and visual goodies like a pet and a banner. The DLC will be available in 2017 but no pricing is yet available. As with other classes, there will be both a male and female option to pick, and the Necromancer will feature many of its classic abilities including blood golems and skeleton summoning - as well as new blood magics.
Later in the day, additional plans for Diablo III were revealed, including an Armory option in town to allow for multiple gear set-ups and a crafting tab to streamline that process. There will also be a couple more maps released in 2017 for players to explore.
Prior to BlizzCon, many had been speculating that Blizzard was remastering Diablo II with Diablo III's engine, which would have been quite a large production on Blizzard's part. The original Diablo is a much smaller game overall and much more sensible to be released as a piece of free DLC and as a fun event for the franchise's anniversary. Nevertheless, expecting Diablo II and getting the original Diablo has made many community members unhappy.
Many had also been speculating that Diablo 4 could also be announced, which obviously wasn't the case. However, there's good evidence to suggest Blizzard is transitioning to a new Diablo project. Much of the content Blizzard has released over the past year, or are planning to release soon, seem likely to have come from a canceled expansion. The implication being that Blizzard is finalizing Diablo III content and moving onto what's next. There was also a comment made at the end of the Diablo portion of the BlizzCon 2016 keynote that more of the franchise was sure to come.
Diablo III's The Darkening of Tristram anniversary event, which adds the original Diablo, launches in January 2017. The Necromancer character pack will launch in 2017, with more details to come.