Few things to make necromancers really cool and not simply a zerg:
Unlimited Undead summon possible but need to require corpses, and will have time limits.
Desecrated ground shall negate said time limit, but have its own time limit.
A multitude of support skills for undead, aka. cadaver explosion, undead rage, life siphon (to caster via killing of own undead), buff undead with DoTs' aka make them plague carriers, and so on (i could literally keep going for half an hour here)
As for a more fun, less diablo esque undead combat, which thrutfully sucks as your stuck to watching your minions slaughter everyone with little to do yourself except spam curses/ more summns, be able to have undead array in formation and only target selected "areas"/ targets so as to actually have any gameplay as necromancer which would also increase their defensive capabilities and make them less swarmy overall
curses to bring enemies down in strenght so as to not make strong undead a necessity, but make them relevant nonetheless simply by making everybody else just as irrelevant
All of this would make necromancer hordes rather tricky to pull off but a possibility and a fun one at that which would require a multitude of tactics and coordination to pull off and not have them simply point at an area which to obliterate.
Not to forget there are like a thousand spell possibilites that have nothing to do with undead minions. Hope there are cool original spells among the necromancy school and please, for gods sake, have everything be skill shots, nothing i despice more than sure-hit spells
Thou seeing how the necromancy introduction video has the caster raise 3 skeletons from nothing alrdy dispenses lots of these ideas.