Long post that will likely never be read by any devs incoming. The things Necro needs are pretty straightforward and could be implemented in a number of ways. What it all boils down to though is simply a reason to use more class skills. Outside of Blastbones and Resistant Flesh, just about everything else in the class kit is a downgrade from non-class alternatives.

Give them class access to Brutality/Sorcery or Prophecy/Savagery. Give them class access to an on demand stun, or at the very least a class gap closer. Significantly buff the spammable, or add a class execute. They have DoT passives but only have 2 class DoTs - both of which are AoE. A class sticky DoT would go a long way. Their damage in PvE and PvP is pretty laughable right now, and they weren't given anything after the Harmony and Graverobber nerfs (which was yet another example of a poorly thought out sledgehammer change, but I digress).

Corpse gameplay is clunky and has only gotten clunkier with time. Corpses don't last nearly long enough. Player corpses should not disappear when the player respawns - the amount of times I go to use up a corpse only to find that the player respawned removing the corpse right before I cast my ability is infuriating. Duration increases to pets means that you can't create corpses as quickly anymore - you should be able to create the corpses at 8 seconds for any ability longer than 16 seconds. Nerfs to the Arcanist/Archer and Mender/Guardian means there's even less of a reason to run those corpse creation abilities. The Animate Blastbones ultimate doesn't even count as a corpse consumption ability, despite consuming corpses being a key function of the ultimate, meaning that you can't even see the corpses you need to use the ultimate on. Tether/Coil and Siphon are good on paper, but in mobile boss fights and most PvP fights, these skills are almost impossible to use consistently.

Necro pets are also stuck in this weird middle-ground between pets and normal abilities. They don't proc sets (neither healing nor damage procs), they don't count for healing or damage on the BG scoreboard (despite blastbones usually accounting for well over half your damage), and yet you cannot command them with the keybind like you can Sorc/Warden pets. This leads to a myriad of problems. The Arcanist/Archer will often just stop attacking anyone, despite enemies being within range. They attack who they want, when they want. The Ghost will also occasionally miss heal ticks because it can't find a target (again, despite there being targets in range). Blastbones is still occasionally drunk, and the fact that you can't recast the skill if it bugs out until it expires can really hurt you in a PvP fight. If Blastbones is drunk, then the ones summoned from the ultimate are dying from alcohol poisoning. You can't tell them who to attack at all (not even on cast) and a solid 30-40% of the time the Blastbones summoned by the ultimate simply expire without ever detonating.

TL;DR: Necro is a hot mess right now. Half their abilities aren't worth the bar space, and the other half are either underwhelming, inconsistent, or buggy. They lack 2 major offensive buffs (damage and crit), lack an on demand stun, lack a gap closer, and lack an execute. They have the worst mobility in the game and a bad class spammable that sees little use in any environment.

Necro offense definitely needs a rework/buff, personally i would make these changes:

Give the class a sticky dot so it can benefit from the class passive.
Give major sorcery/brutality to both arcanist/archer.
Change colossus ultimate to be target cast so you can't miss it and make it cheaper (i would make it at least 200).
Make scythe execute.
Give the class a reliable stun on demand.
Make the spammable actually useful that synergies with the class in a way like dk whip.