Demon Hunters were established as hunters of demons who could turn into demons, not spell casters who keep demons as pets. This got a bit muddled because Warlocks got the ability to transform into demons and it reduced the design space for the Demon Hunter class to exist. However once that was changed you could bring a DH class into WoW.
In the case of Necromancers and Death Knights, that difference in theme doesn't really exist. The only difference people can come up is a range difference, and since nothing states that Necromancers need to be ranged, that difference point is irrelevant.
Again, nothing specifically states that a Necromancer needs to be a ranged spell caster. Also Death Knights also are spell-casters.Necromancers and Death Knights are different archetypes no matter how you try to spin it, they are distinct in gameplay (ranged spellcaster vs melee fighter) they are distinct in theme, Death Knights are fallen heroes, raised into unwilling servitute, who were freed and wield the dark powers they were granted meanwhile Necromancers like Kel'thuzad willingly studied necromancer for the sake of power, ambition or knowledge, they are distinct archetypes the Death Knight is not a Necromancer class it is a Dark Knight/Anti-paladin class, they are distinct archetypes no matter how much you conflate them, Kel'Thuzad isn't called a Death Knight hes always been called a Necromancer.
Because the main purpose of a Necromancer is to summon undead minions, something Death knights already do. A Necromancer is a character that uses necromancy, that's it and that's all.If Demon Hunters can exist as a fighter equivalent of Warlocks despite using the same type of magic (fel) because of metamorphosis, melee fighting, mobility why can't Necromancers exist as a spellcaster equivalent to the Death Knight class, theres no rule that Necromancy in WoW can only be represented Frost, Unholy and Blood especially now that Shadowlands has greatly expanded on Necromancy and if a Necromancer class had access to different forms of Necromancy (like poison or anima-based spells) why can't they exist on the same paradigm as Demon Hunters and Warlocks or Paladins and Priests who share forms of magic.
Again, the Priest class exists in WoW as a way to have a spell caster that uses both holy and shadow magic. It serves a rather distinct purpose in the class lineup, and is frankly the only class that can do it. Creating an inverted Death Knight that has a healing spec that uses blood, is ranged instead of melee, is cloth instead of plate, and other silly things to create a distinction for the sake of distinction is rather pointless.No fan necromancer concept i've seen just retreads Frost, Unholy and Blood without changing them, theres are plenty of things DK's don't do or aren't massive parts of the DK specs identity, Insects, Poison, Corpses, Anima, Contructs, Bone. Even things like Blood Healing are different expressions of the same theme (Blood Magic) and if Blood tank spec prevents Blood healer spec from existing why do things like Holy, Protection and Retribution Paladin exist when Holy Priest exists? if "Blood Tanking" is enough ways "Blood magic" can be expressed why do we have 4 specs that use holy magic in different ways shouldn't Holy priest be enough ways to express "holy magic" following this logic? why can't 2 specs share Blood magic but 4 specs can share holy magic?
Also given that Kelthuzad is the only major Necromancer character remaining in WoW, there's no way you're creating a necromancer class without a Lich specialization. Unfortunately in order to do that, you'll have to gut Frost DK and Frost Mage.