I was thinking maybe +5 levels to corpse maximum, (for level 56 max zombies, similar to strong npc zombies but not quite as powerful as the best ones) and then add some cool flavor stuff.

If you killed fortress guy yourself and then raised his corpse, and fought evil characters with the zombie, he occasionally prog some sexy fire echoes and do fire damage instead of punch, or .

If you killed scion mage yourself and then raised his corpse, he prog interesting shadow echoes on command and gives the necro a buff from the shadow plane - maybe extra dam redux against negative or something that is very interesting from an RP perspective and very limited from a PK perspective.

You could also add drawbacks to certain zombies, too. Fort zombie explodes in a cascade of flame after a certain time with the necro, blasting everyone in the room. Zombie of a thirsted rager will eventually start attacking everything you walk past.

Stuff like that. It would make player zombies more interesting but not overpowered. You'd still use unicorns or whatever if you wanted max power, since you can do better than level 57 zombies, but you could get some cool RP opportunities and flavor from each corpse that might outweigh small power considerations.