Classes have a "what" that they do, and a "how" that they do it.
The "what" is the role question. Do they funnel damage away from the party, lay the hurt on individual enemies, debuff/disrupt enemies, or buff/heal allies?
The "how" is a style question. Do they do their thing in melee or at range? Do they use weapons or implements? What defenses do they target? What status effects do they inflict? How are their abilities described?
The fundamental aspect of the Necromancy concept is of a evil (or at least 'morally challenged') spell caster who creates undead monsters to do his bidding.
That's great and iconic and totally worthy of getting its own class, but that's all "how" and no "what". A true to concept necromancer class could easily be built that does any of the main roles, as undead minions can be used for each of them.
Necrofender? Con based pet class with a single tough pets, or multiple wussier ones, with life draining spells that grant him temporary hit points and a marking ability that lets his undead buddies slap enemies who attack targets other then the Necro or his minions. The buddies themselves are easily destroyed, but the necro can keep them up by channeling them some of his hit points when he takes damage.
Necrostriker? Pet class with single, high damage pet, or multiple wussier pets and powers that let each of them make attacks with focus fire possible, supplemented by single target death spells combining high ongoing damage and a disruptive save ends status effect.
Necroleader? Pet class with one or more pets that attack for minimal damage but apply penalties to Defenses and Saves. Necro grants temp HP and saves to allies via necrotic energy, and channels HP from enemies for true healing.
Necrotroller? Multiple pets and/or summons acting as unliving walls and allowing the necromancer to strike multiple targets simultaniously. Supplemented with status impairing zones of dark magic.
So yeah, the necromancer could be anything. I'd personally if the necro were not a stiker or leader, since we already have pet classes for those. Of the remaining two rolls, I think controller is the best thematic fit, but that's just personal opinion.