I've been consistent on this for years. Necromancer = Warlock Glyph.
The archetypal Necromancer is a light armored, plague and disease wielding, undead raising
magic caster. DK is a melee fighter first, so it's not an appropriate substitute. DK is the Paladin to the Priest. It's much cleaner to just do a Warlock enhancement vs design a completely new class and have to balance more specs or try to force a DK into becoming a caster via a 4th spec.
First and foremost.... mechanics. Warlocks are mechanically inline with what a Necromancer is. Primarily a magic user, one who uses dark magic to raise undead, spread disease and rot their opponents with strong DoTs. We already have that foundation built with the Warlock. What's nice about this, vs a DK 4th spec is instead of 1 spec, you now have 3 specialized Necro specs:
Demonology - Demons =
Undead, and the spec becomes a Necromancer who focuses on controlling an undead army as his or her primary expertise.
Destruction - Fire =
Frost, destro would become the equivalent of a Lich, and let people explore a Kel'Thuzad-style Necromancer.
Affliction - Shadow/Void based DoTs =
Blood/Disease based DoTs, this is the DoT based Necromancer who would prefer to rot away opponents with disease and poison.
Shadowlands would be a perfect time to add something like this. I'll list out some examples of specific changes I think would happen, and these art assets already exist in game and many more options are being added in the upcoming expansion.
Your Demons will convert to Undead:
Voidwalker (tank) - Skeletal Warrior
Felhunter (interrupt/dispell) - Skeletal Assassin
Succubus (cc/invis) - Skeletal Archer
Felguard (AOE dps) - Abomination with radiating plague damage.
Grimoire of Supremacy can jack up all of those base models to use some of the really cool Shadowlands undead minions.
Void/Shadow magic and Fel/Fire magic becomes Blood and Frost:
Incinerate - Icy Bolt
Immolate - Deathchill
Cataclysm (Talent) - Death and Decay
Corruption - Plague
etc
Some things still fit just fine:
Drain Life / Soul
Life Tap
Ritual of Summoning
Curses
etc
My end argument is that I view a Necromancer as primarily a magic user. And I view Warlocks as a Mage who will use forbidden magics. I simply view Necromancer as a specific TYPE of Warlock. A single glyph that modifies the flavor of your spells basically completely fulfills the fantasy.