A lot of questions and issues would be solved if Necromancers got an expansion focused on them or similar development. I'm speculating it could happen two or three expansions after Dragonflight personally. Post-Northrend Campaign, we know the Cult of the Damned broke away from the Scourge and tried to take over Western Plaguelands. They were forced to retreat to Scholomance after getting double-teamed by Koltira and Thaurissian. We later go deal with them ourselves in the revamped Scholomance.
It could be explained after Scholomance was besieged by agents of the Argent Crusade, most of the Necromancers who survived were rounded up and locked away in the rebuilt Hearthglen south of Light's Hope Chapel. Some who managed to escape went into hiding within the Alliance or the Horde depending on their race, some meeting people interested in learning Necromancy from them. Several years later, the veil between the Mortal Realm and the Shadowlands is shattered. The Necromancers in hiding as well as those who trained under them find themselves empowered as a result.
After the Jailer is dealt with, the Knights of the Ebon Blade sense the surge in familiar Necromantic power. Bolvar is about to send knights to seek them out when they are approached by emissaries of the reformed Cult of the Damned long believed to be wiped out. In exchange for the Ebon Blade's protection and their comrades being released, they offer their skills to both the Ebon Blade and anyone interested in learning their craft. The Argent Crusade objects at first with good reason until Bolvar reminds them technically, every Death Knight is a Necromancer. The Kirin Tor and The Black Harvest both come forward expressing interest as well as offering to help keep tabs on the Cult of the Damned.
Necromancers will have three specs: Bonewalker (Tank), Deathweaver (Caster DPS) and Grave Warden (Melee/Caster DPS).
- Bonewalker (Tank): The Necromancer trades commanding the Undead for the power to empower themselves, becoming capable of absorbing heavy punishment for allies. Instead of raising Undead, Bonewalkers harvest the bones of the fallen to reinforce its defenses as well as refine its attack power.
- Deathweaver (Caster DPS): Your typical Necromancer who can raise, dominate and control Undead. Powerful enemies can be raised after slaying them to fight under you. Deathweavers can control up to 10 different Undead at the same time.
- Grave Warden (Melee/Caster DPS): The Necromancer infuses themselves with the powers of a dead warrior you've encountered with devastating efficiency. Unlike Deathwavers, the Grave Warden can't have more than two Undeads with them at a time.
The main thing for the DPS Specs is mitigating damage to the Necromancer via the Undeads they control. The Tank Spec is about absorbing enemies and restricting their movement. It's a Hero Class as well. Necromancers use Leather armor on that note. Most races and all allied races can get the Necromancer class. Necromancers get a unique land and flying mount. For Legion Timewalking, Necromancers do get artifact weapons and a unique class hall just for them with the help of the Bronze Dragonflight. They acknowledge they are looking at past events they were not present for at the time.
Grave Wardens have a Signature skill called Echo of the Departed which allows them to infuse themselves with strength and fighting abilities of a dead warrior from the past. Varian Wrynn, Garrosh Hellscream, Varok Saurfang, Vindicator Maraad and even Arthas Menethil are a few such characters whose powers you can make use of. Each subject involves a quest chain to use and some are faction exclusive. You may not be as powerful as the originals but your foes won't know the difference. The more you harness a particular person, the better at using the powers you get. You even learn some never before info about them as well.
That's where I'm at personally. I now think it's possible to pull a whole class out of it without altering DKs. DKs are basically plate-wearing Necromancers but their using Necromancy is what makes them extra special. A separate Necromancer class that is all about using death and corpses as their plaything needs to have unique skills and abilities to really stand out from DKs. That's why I came up with those specs. Necromancers can use Regents that drop from enemies to use some of their abilities when corpses aren't available in some areas as well.