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.
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
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
Speculation
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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.
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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.
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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).
Base on Vaeflare’s wording in the first sentence I think that’s more of a reference regarding Mehtan the Necromancer (also the apprentice of D2’s Necromancer) who can be found in D3’s Act II. However, not everybody may have not of heard of him since he was one those randomized NPCs that would show up every now and again. But if were another character more established in the Diablo mythos… I would agree with you on Zayl since he’s already visited Westmarch and would have reason to comeback.
Eroldren I have a feeling the next expansion after Reaper of Souls is taking us to the Necromancer region. Notice how Diablo III was New Tristram –> Caldeum –> Dreadlands. Now this expansion is Westmarch –> Bloodmarch —> Pandemonium Fortress. Next expansion might be the Swamplands and Ureh. Seems they are going to focus the expansions on specific regions. Take a look at the map of Sanctuary. Where Diablo III locations are, and where Westmarch is in relation to Tristram.Â
 https://media.blizzard.com/d3/media/screenshots/guide/global/world/intro.jpg
medievaldragonESPÂ EroldrenWell, unlike the SC2 or WOW universe that we’ve been told or
can see the plot direction leading those two franchise can go, the wherever Blizzard
wants to take Diablo 3’s story entirely been kept under wraps. Who knows really
what their end goal is.Â
Although I would like to the see the Naphalem going eastward through Duncraig, Kingsport (while also tying up Lyndon the Scoundrel’s story) and the some parts of the Skovos Isles before we go entering the jungles of Kehjistan. That part of Sanctuary should definitely have its own dedicated expansion (on a massive scale; D3-size).Â
And come to think of it after thinking of Lyndon, I wonder if the mystery of the unknown angel Prophet of the Enchantress’ sisterhood could be explored in this area if the Templar’s own back story in Westmarch that could also be a potential plotline explored in Reaper of Souls. That’ll be utterly fascinating.