Monster Sets:
Slimecraw - for crit and minor beserk, will be the best choice in solo pve and in trials, like vAS, where uptime of combat prayer is pretty low.
Iceheart - for extra survivability and crit damage.
Zaan - best damage proc set for now, also gives you crit.
Balorg - for shot combat burst damage.
Grothdar - for aoe builds, you may use that, but maybe you can benefit more from Slimecraw. (Need to be tested)

Sets:
Mother's Sorrow - is best for critical damage so far, but if have no one, you can use Law of Julianos.
Spell Strategist for great single target damage, but also can replaced with Burning Spellweave without loosing much damage. From last set you will also great benefit from aoe damage.
Perfect Siroria is great set for trials if you keep uptime well. Best for the spd.
False God Devotion - new Elsewyr set, equal to Vicious Ophidia, but for the mages. Can be really good in dungeons and Maelstrom Arena.
Spinner + Mother's Sorrow for aoe build with lightning staff on frontbar to get aoe penetration, but can be not that usefull because of graveyard and necromancer passive that gives you 6500k resistance reduction. (Need to be tested)
All traits are divines. Backbar fire maelstrom - infused, and mainbar Mother's Sorrow/Julianos - presice.

For Maelstrom:
Mother's Sorrow + FGD + Slimecraw or Iceheart if you wanna drop shield.
I would prefer to use spirital mender insted of skeletal mage, to have healing, and beckoning armor to get major resistances and pull enemies to you, making destroying ranged mobs easier task. You can put it instead of Empowering Grasp, so you will not loose your sheild and major force.

This build is just theorycraft for now, because we don't really know, how necromancer will perform in real fight.