I've given the Leige-Kavalos a good long look, and I might be better served to find a way to bring OBR more individuality instead of shoehorning in a vampire. I would need to either to an extraordinary amount of greenstuff sculpting or have the figure in classic vampire armor, neither option appeals to me.Yeah it is a conundrum that they are monolithic and that is both their appeal and their deterrent. I would think however, that with time, just like robots (in most sci-fi settings) develop bugs that then act as personalities. Or a certain Legion might have a weird interpretation of Nagash's will.
As you mentioned Sun King, there is some precedent for imperfections in the process of soulmasonry, the account cited in the Battletome was a Soulreaper that suffered so much anguish it had to be given a body without a mouth (as far as I remember). You could get a vampiric effect, accidentally or intentionally, going that route, and have a Bonereaper that must satiate its hunger for souls, allowing for a more flawed character, but it feels weird on a Bonereaper and would likely wouldn't give it any more breadth. After all, the Bonereapers are already driven by the hunger of Empire.
My thoughts turn back to the Vulture Lord, who was crafted into a Bonereaper from a single soul, like Katakros. Except the Vulture Lord's loyalties to Nagash are more transactional, he does Nagash's bidding because he gets something tangible out of it, and has his own agenda. Indeed, two of his subordinates are likewise 1 to 1 soul conversions. What might happen if the Vulture Lord was destroyed? Would these undead minions still abide by Nagash's bidding? And as mentioned previous the Cremetorians are basically subverting Nagash's design in their quest not to be blown to oblivion.
I may just wait until the presumed new foot hero is revealed before making anymore army background decisions, after all Mir Kainan got me into the faction, and if the character is a generic Executioner I would be very inclined to explore the thematic possibilities of that.
Otherwise, I've started in on assembling my second set of Necropolis Stalkers, and I hope that they continue to be battleline for Mortisan generals in the next book. Would love to run Petrifex Elite with a properly tagged Executioner. Of course, if we get the Nighthaunt treatment, which is likely, who is to say what will open up gameplay wise.