Which units would you like to replace/add?

I really want to add Death Knights to the Necropolis faction. I just want to make them stronger so they are the coolest units at level 6, just like in Heroes of Might and Magic III. As for Death Harbingers, they are certainly in the style of a Necromancer, but they are not 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. They are all created from humans. But how do you make a Death Harbinger from a human?