And yet unique gameplay. That's all it takes.
There's nothing about a frost mage that would be out of place on a Frost DK, or either of Priest's holy specs that would be out of place on a Paladin.
There's nothing about DH that would be out of place on a Warlock.
What's important isn't shared thematic, which is everywhere (Mistweaver/RestoSham) it's about how the class stands out mechanically and rotationally. There isn't anything like Necromancer in the game currently. There are just a couple classes that share thematics.
Nowhere. We don't need more tank specs, and Necromancers don't traditionally tank. I was just pointing out how the class can easily be constructed without stepping on the toes of DKs or Warlocks.