Vaeflare and Grimiku confirmed in the official Diablo III forums there will be a known Necromancer NPC in the expansion. There are few Necromancers that can actually be a reference here for someone we are familiar with.
[bluepost]Vaeflare: If you're keen to see what the Necromancer has been up to since the events of Diablo III, you'll be pleased to know that you'll have the opportunity to meet up with him once more in Reaper of Souls!
Also as Josh Mosqueira stated the interview with King Konger and elsewhere, we do not currently have any plans to add another class to the expansion. We feel the Crusader class is a truly epic, and fitting addition to Reaper of Souls, and we're excited to share more about it with you in the months ahead. :) -- source[/bluepost]
[bluepost]Grimiku: Vaeflare confirmed it earlier, but there is no harm in saying again. You'll get a chance to meet a familiar necromancer npc, but the only additional playable class in Reaper of Souls is the Crusader. We'll have more information to share about the Crusader in the coming months, but this armored, battle-worn, veteran will be a mighty addition to hero roster of Diablo III. -- source[/bluepost]
Nameless Necromancer: This is the playable Diablo II Necromancer. To my knowledge none of the Diablo II playable classes had a name. However, the nameless Barbarian in Diablo III is the same we met as a playable character in Diablo II. We got to meet the original heroes of Diablo I in Diablo II actually. The Rogue in Diablo I (member of the Sisters of the Sightless Eye) was actually Blood Raven in Diablo II (Act I). The Warrior in Diablo I turned out to be The Dark Wanderer (aka Leoric's son: Aidan) in Diablo II. Lastly, the Sorcerer in Diablo I turned out to be The Summoner in Diablo II (Act II, The Arcane Sanctuary). Thus, it might be possible for the Diablo II Necromancer to make a cameo in Diablo III: Reaper of Souls.
Zayl the Necromancer is a well-known necromancer who has never been in-game. He has appeared in Diablo: The Kingdom of Shadow (reprinted in the Diablo Archive) and Diablo: Moon of the Spider -- both by Richard A. Knaak.
Kara Nightshadow is another well-known necromancer from the pages of Diablo: Legacy of Blood (also reprinted in the Diablo Archive). She helped Norrec Vizharan (the mercenary treasure-hunter who found the armor of Bartuc the Warlord of Blood).