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 mechanics or try to force a DK into becoming a caster.
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. You don't have to design a more spell focused spec for DK. You just reflavor the Warlock, and each spec becomes a specialized Necro:
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.