Some variation on the character-lore background-traits (that you pick 1 of and locks other) could be neat? Like things more focused on the Nagashi backgrounds that replace the existing mortuary-cult ones the regular Liche Priests get?

Eitherway, these are intended to be rather powerfull if the Chaos-Warrior's hero ones are any indication of more recent hero-traits.

"Mortuary-Cult Infiltrator" - Not sure if possible, but it'd be neat if it enabled the ability to steal-research, which is something Tomb-Kings normaly cant do, due to how powerfull it'd be, otherwise it'd probatly lower-cost on mortuary-cult stuff and grant research bonus.
"Study of the Dark Arts" (i imagine it'd give you x number of raise-dead (the bound spell) + the dead raise again chance+increase heal for army)
"Warpstone-Touched" - Magical+Poison Attack, Passive Ability: Warpstone Detonation, (sorta inspired of the fact that Nagash Factions harvest Warpstone for magical-use to increase their magical potency)
"Keeper of Dreaded Tomes" - Chaos-Shrine of Tzeench's passive ability that grants scaling spell-power at increasing ammount if enemies nearby character dies, +Leadership to nearby units aura.
"Soul-Harvester" - Missile-Strength & Ammo for Self/Casket of Souls in Army (intended to buff self when on casket, and caskets of soul in army)
"Sprit-Host" - Character gets Frostbite-contact effect, -20% fire-resistance & Horrible-Regeneration for Hexwraith/Cairnwraiths in Hero's Army. (fire-resistance just offsets fire-vurnerability)

Thanks - I'll take a look at this!