Hum...skills working with corpse would probably be, because of their fast disappearing in RO, (de)buffs that would do something upon target's death. Corpse explosion...meh, would be fun, just use it on enemy paladin and bring him down...KABOOM. Wipe.
Oh yea, D2 skills...they'd be fun, like bone spear, frozen orb, etc...imagine those at WoE.
Getting these to game...they'd have to go through usual 1st class-2nd class (trans)-3rd routine, so they'd be as powerful and as slow to level as 3rds, to make them playable but not so easy to level. Perhaps branching them as another "main" classes from normal 1st, like...mage->necromancer->high mage->lich->great lich.