Which units would you like to replace/add?

I really want to add Death Knights to the Necropolis. I just want to make them stronger so they are the coolest units at level 6, like in Heroes of Might and Magic III. As for Death Harbingers, they are certainly in the style of a Necromancer, but they still aren't Necromantic... they are also not undead and cannot be resurrected as such. And you can't negotiate with them or accept them into your army. Skeletons, vampires, ghosts, and liches are all native to the Necropolis and are made from humans. But how do you make a Death Harbinger from a human?