I've always taught about necromancers summoning dead mobs that THEY killed.
Section 1---------For example: Killing 3 mobs, casting the 'rising the dead' skill on them (adding luck mecanic to the skill, 1rising success =>CDgoes 1/3 ,2 rising success =>CS goes 2/3, 3rising success CDgoes 3/3)
Section 2----------then you can also add some balance in this, example: (I rised succesfully on normal mob, then my minion has 85%-95%% of his stats; i rised succesfully one mini-boss i get a minion with 50%-75% of his stats, i rised succesfully one boss i get a minion with 25%-30% of his stats) . then you might do some other balancing with conditions like only summon bosses /mini bosses with ability 'rising the dead that can rise X normal mobs.
Section 3---------You may also add purchasable ingame currency pacts/contracts/scrolls; What i mean by this is when a necromancer player finds a special mob or set of mobs (lets say 1goblin 1skeleton and 1mini-boss orc ) then he can buy 3 scrolls to make a permanant contract with them wich makes him summon them without having to go kill them or farm them each time. we can do common scrolls for normal mobs, rare scrolls for mini-boss, ancient scrolls for boss. maybe even make scrolls stack in inventory in case the player has several tactics or sets or customizations for his gameplay flexibility.
Section 4----------Maybe also play arround a set of skills that interacts fully with those summonnings, example : a skill for commanding them to : ATK , Deff, AGRO, Folllow , Retreat . Maybe sacrifice a mob for health or mana or dead power (either the mob vanishes or get on CD if under contract). Maybe also add a loyalty system for mobs under contracts (they dont want to follow you when you sacrifice them X times). Adding a TP mecanic for retreat or ambush or even obstacles.
Section 5------You can also do minions formation that depends on the set of minions you have summoned:
example; you have all orc type minions or a special mixup of races/classes/elements of minions. Lets say you have a set of ORCS then you can have the skill ‘Bone Cage formation’: you make from the orcs bones a cage, if enemies are trapped in, they get feared and poisoned from the miasma emanating from inside your cage, and then you can only summon those minions again if you kill X mobs. You can also have a mix of water element and fire element minions wich gives you a ‘mist’ formation : your water minion and fire minion charge on the enemy and explode dealing AOE water and fire elements after 2 secs a mist emerges that can heal and have movement speed and CC immunity while enemies get confused and poisoned from the miasma that traveled through the mist etc…..
Section 6------------ A necromancer plays with death, why not make him interact with dead players. he can semi-revive players and give them a second chance
example: the warrior of the group died, the necromancer has 5 seconds to activate his skill 'nocromancer pact'; where he revives the warrior, the warrior has all his skills but can only live if within the 20seconds he gathers enough life force through kiling or damaging, if he does, the warrior stays alive for Xminutes (depends on the skill 'necromancer pact' power or lvl), within this time, the necromancer enjoys the warriors life getting drained and converted to mana/death power, until the warriors die for ever :C .
Section 7--------------Because necromancer is the death symbol, letting him die normally isn't enough sense to me, why not make him this:
when he reachs 0HP, the passive 'Beyond death' activates,wich makes him consume death power or mana instead of hp to live , his skills dont consume anything but cd gets a little bit longer. if he can make his death power/mana to full, he gets half his hp back and 0 death power. To make him worthy of his reputation.