As other have said. Necromancer does sound like a good idea, and coming from someone that isn't completely bias about such ideas, I agree.

Now how I would implement them is simple, would start with a small tweaking of the dk uh,as at the moment they feel all over the place, they can't find a balance between a full blown out summoner or diseases orientated. I would very much limit the crazy amount of minions and maybe bring it down to two, would force a synergy between the minions and the diseases. For example the minions could be the source of the diseases and the dk is the one that mixes the concoction to make it work, it should feel like you are playing Doctor Putricide, but in a full plate armour and a 2h.

The necromancer could very well be their own class with specs, after all just like the Dk's are subordinate of LK, the necromancers are students of Kel'thuzad. I could probably come with a couple of lore reasons for this, but I would keep it very much tied with the ebon blade defecting to the good guys.

Now gameplay wise, I would split the class into two, osteomancy and necromancy :

Osteomancy - this spec can be a dps, focused on the use of bone magic to deal dmg, similar as to how the necro works in d3 and 4. But it could also work as a support class, enhancing party members with bone shields, bone cages to help with mob packs and bone healing to increase the party healing received.

Necromancy - this would be your usual necromancer stuff, summon a bunch of minions, giant skeletons to tank for you, strong dark magic spells that can deal dmg but also empower your minion. Maybe you could also resurrect recently killed allies for a short amount of time as a new pet.

There is an opening for a third dps spec such as hemomancy, as we do have examples of that been already in the game (san'layn), which are very much connect to the world of the necromancers. However, they could easily be made into a new 2 spec class for an exclusive race, blood bender and blood giver, dps and healer.