Blizzard game development team added only one new class to Diablo 3 new expansion, which made players feel a little disappointed. Though the expansion seems exciting and wonderful enough, a crusader is not that enough for fans after so long waiting.
Many people would like to see Necromancer back in Reaper of Souls, because this class is powerful enough by controlling the dead, which is similar to the death angel Maltheal. The community manager has posted to prove that Necromancer will back as NPC in the expansion.
Necromancer tends to keep the balance between life and death, if the death army is too strong and desire to control the world, they would appear and deal it with their own way. More details about the Necromancer still need to waiting for Blizzard official news, maybe we can know it at BlizzCon.