The Necromancer class was previously considered, but didn’t make it to the original roster of classes. Future expansions might introduce the Necromancer class. However, that is not guaranteed.
Nethaera: “We have many interesting classes that we could implement, but like all things, we needed to decide which classes would be the most interesting and capable of being robust additions to the game from the start. The Necromancer is one class that has been in consideration before. Who knows. Perhaps someday the Necromancer will come into play within the World of Warcraft, but that is something for the future to decide.”