Dark Souls

Wouldn't be hard. Just allow the player to resurrect killed enemies and have them act in a way similar to a foe under the effect of Undead Rapport. Change the "Return to hostile" timer to a "Die" timer. Anything that fades cannot be resurrected.

All a Necromancer would have is a less useful version of Undead Rapport. What's the point of beating something down to use Necromancy if you can just use Undead Rapport to functionally do the same thing?

They aren't going to put a pet class of abilities into the game because "hide behind summons and run away when something looks at you funny" becomes a viable strategy. For a game that is going out of its way to be difficult, that is not a playstyle they are going to encourage. So, if necromancy was included it would be temporary and largely uncontrollable to remove that playstyle, which Undead Rapport already does.