The Sil'dih were able to create a Zombie army, so if Necromancer was to become a Class/Job this is where we would potentially learn the art. (Sil'dih empire may or may not be completely destroyed). I'd say Lore wise to justify learning Necromancy it would definitely be a Dark Art, Thaumaturges commit assassinations and steal valuables from dead bodies and even they think Necromancy is a bad thing.

As for game mechanics wise, it'd be fun I guess. The tough part would be balancing it with Summoner and Beastmaster(which is probably coming sooner rather than later). Summoner looks as if it will be able to DD along with Support and Debuffs, so it'd be tough for Necromancer to do similar things while being different.