I would like to see a Necromancers guild as well with a good story and interesting characters.
But could you come up with a good event\story for it? Would have to be something with dead bodies, rituals? Blood, bones, all that necromancer stuff.
Well, graveyard looting, buried chamber pillaging, secret entrances and a secret cult or more refined necromancers than your average necromancers would be a start. An example of the dark rituals that could happen is something similar to the gatekeeper quest of the Shivering Isles DLC in Oblivion.
Some deeper purpose that'd be revealed to the player after they earn their trust. Perhaps to zombify a figure in a high position, to gain control of an entire city? Kinda like a thieves guild meets dark brotherhood, with a hint of mages guild, but highly illegal and evil.
Naturally the DLC would have to include an equally interesting anti-necromancer group. (vigilants of stendarr's reformed?)