Blizzard will offer Diablo III players a new Necromancer character in the Rise of the Necromancer Pack which can be purchased next year. The Necromancer character last appeared in Diablo II.
During BlizzCon 2016, the developer revealed the new character with the Rise of the Necromancer Pack reveal.
To be able to purchase the Necromancer, you will need to own the base game and the Reaper of Souls expansion, or for console owners, Diablo III: Ultimate Evil Edition.
Also, next year, Blizzard will be adding two new zones for Diablo III: Shrouded Moors and Temple of the First Born.