So, I had an idea while leveling my warlock and death knight and realized the biggest problem (for me) with adding a necromancer was the concept of originality. How could you make a caster class different enough from the warlock or mage to actually warrant a whole new class with different abilities. Its possible but it would be difficult to make it unique enough without letting the others feel like their class abilities were stolen.
That’s when I had the idea of making necromancy a subsection of different classes. The most obvious would be death knight, but that would be the first time a class would have to collect two different armors.
There’s mage, where you could add in an actual pet subclass for mages instead of how frost acts with using the frost elemental as a cc proc but there’s nothing in the game to make the mage class feel like an obvious answer as they don’t touch death magic whatsoever.
Then there’s warlock. Here’s where things get tricky. They meet all the requirements, except lorewise they exclusively use demons (which isn’t necessarily a problem as there are demons in lore who use necromancy so borrowing their power would explain most of it) but more importantly, they already have 3 dps trees. Adding a 4th would be ridiculous. You could have necromancy be the first warlock healing path, but that doesn’t quite fit the conceptual ideal.
All that taken into account, I feel like mage and death knight are the most viable. Death knight for obvious reasons, other than the con of collecting a different type of armor. This would make the necromancer a completely unique casting class that uses spells like frostbolt to generate runic power, and use them for a wide range of abilities. This is similar to how warlock uses soul gems, but as stated earlier, giving warlock a fourth dps tree would be ridiculous as necromancer should add something new to the class they join. To make things more interesting they could give necromancer all 3 runes again to allow them to cast other death knight abilities like howling winter. Mage would also work, but as stated with warlock, they already have 3 ranged dps classes.
Making necromancer a subclass of death knight would allow death knights to have a ranged dps class. They have 2 melee classes already but neither choice really changes the part you play in a group or raid.
The other alternative is finally biting the bullet and bringing in hybrid classes that use other class abilities.