It would depend on what Blizzard wants to do with a Necromancer.
Did they change Holy Paladin and Holy Priest at all when they were both designed over the 15+ years that we've had both classes? I remember there being times when Holy Paladin had no shielding abilities and Holy Priest was unique in having Divine Shield. Then during Wrath they gave Holy Paladin Sacred Shield. Then there were subtle differences with Holy Priest having AoE heals while Holy Paladin had no AoE heals but had Beacon of Light. Then later Holy Paladins were given Holy Radiance. This is an example of two classes having specs that became more and more similar to each other.
With Frost DK's, they used the Lich as a core inspiration for the spec. But at that time, Frost Mage already had iconic WC3 Lich abilities Frost Nova and Frost Armor. What happened to DK's then? They got their own AoE frost spell through Howling Blast, and instead of Frost Armor they got Icebound Fortitude. Both of these spells are much more similar to how Frost Nova and Frost Armor worked in WC3, and they didn't have to be removed from the Mage at all. This is an example of two classes pulling from the same sources, and diverging with two different adaptations of the same ability types.
I was actually surprised that Blizzard removed the Metamorphosis entirely from Warlocks. I personally don't think Demonology was redesigned because we couldn't have two Metamorph specs; I think it's fully possible. Keep in mind too that there are some behind-the-scenes shenanigans at play too. Most of the Metamorphosis Warlock and Dark Apotheosis gameplay was the brainchild of one zealous Warlock designer who was very vocal about his design. He didn't really care much about lore and he designed the Warlock more on 'rule of cool'. He was kicked out of Blizz around the time of MoP/Wod (I think) and Legion was a time when the designers felt a whole bunch of classes/specs needed to be redesigned to fit a better class fantasy.
https://wow.gamepedia.com/Alexander_Brazie You can read a bit more about Xelnath here. He has his own blog on Warlock design that's still up for viewing, and while Blizzard does design classes as a team, this guy is commonly given credit for making Warlocks pretty cool to play after the first few years of 'Growing pains' of trying to find a Demonology gameplay that was fun to play. If you played a Demon Lock throughout the first 5 years of WoW, you know what I mean. They had a major identity crisis every expansion.
Legion was when Blizzard decided to take a good long look at specs and 'bring them closer to class fantasy'. I didn't agree with all of their changes here. I mean honestly, it doesn't make sense to me that with all the melee specs we already have in the game, they decided to remove a Ranged spec from the Hunter and make it melee. I can't speak on Blizzard's behalf on why they feel this makes sense, because it doesn't make sense to me. Removing Metamorphosis from Warlocks didn't make sense to me either considering the new DH forms look totally different from Shadow Illidan form that was already built into Warlock lore through Green Fire questline.
We have both examples of classes with specs that overlap with others, and classes that had to be redesigned to fit new ones. The answer is both, depending on which classes you're talking about and what Blizzard would view as a Necromancer class. IMO I don't think any new class will be added to the game and I think Class skins are the future.
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Class skins would work, and doesn't need to be based on any expansion theme. Allied Races are already added despite having little-to-no relation to the BFA Alliance/Horde war, especially when regarding Highmountains and Nightborne siding with Horde despite having good relations with both factions during Legion.