Just my thoughts on how I'd like to see a Necromancer played.
TLDR: A couple gauges. You cast debuff/drain spells that build up Necromantic Energy which you spend on spawning/buffing minions. Then as minions die, you gain bones, which you spend on direct attacks.
Gauge 1: Necromantic Energy
Gauge 2: Bones
Life Drain: Cast this DoT on an enemy that drains their life essence, turning it into Necromantic Energy.
Necrotic Spike: Cause Life Drain to flare up, dealing a % damage of Life Drains total damage instantly. Provides a quick boost for Necrotic Energy, the less you have the more it provides.
Essence Sap: A small DoT that also reduces the damage dealt by the enemies normal attacks. Also grants Necrotic Energy (Maybe also make it so Spike also effects his).
Raise Undead Warrior : Raise a skeletal minion that charges after the enemy, attacking them in melee. Raising an Undead Warrior costs more for each Undead Warrior active. Only lasts for a specified duration. When an Undead Warrior expires/dies, it adds to your Bones meter. (Cost Necromantic Energy)
Raise Undead Archer: Raise a skeletal minion that charges after the enemy, attacking them in melee. Raising an Undead Archer costs more for each Undead Archer active. Only lasts for a specified duration. When an Undead Archer expires/dies, it adds to your Bones meter. (Cost Necromantic Energy)
Raise Bloated Corpse: Raise a corpse that charges into the enemy, belly flops, and explodes doing AoE damage. Also provides bones. (Cost Necromantic Energy)
Raise Ultimate: I'd like some neat ultimate after summoning so many undead minions. Either a big bad undead, like a Lich or Death Knight, or maybe the choice between the two. A giant hand comes up and slams down on the enemy. Something flashy and cool.
Bone Spears: Create two portals into the abyss, each spawning a Bone Spear that launches towards your enemy. Chance on hit to refresh "Drain Life". (Costs Bones)
Bone Barrage: Create multiple small portals into the abyss, sending out multiple bone spurs at the enemy. Chance to debuff enemy with bleeds, and slow. (Costs Bones)
Bone Graveyard: Cause a multitude of bone spikes to shoot out of the ground in an area. For each enemy hit, there is a chance to proc a free + instant cast Bloated Corpse. (Costs Bones)
I would like to add a few extra actual debuffs to the enemy that could have the Necromancer be kind of a reverse Bard. Instead of buffs, they debuff the enemy. I just can't think of any off-hand.
LIMITED PLAY: The Necromancer can create a permanent Undead Minion from certain enemies they have killed. Only one can be active at a time. And only can be active where Chocobo's can be active. Basically adds a collection akin to the Blue Mage, and some limited playstyle to it, while still being a full class for dungeons/raids.