Sorry I'm late to the party, but stumbled over here whilst researching how to write new playbooks.
My two cents is that the axis paradigm is solid, especially when coupled to protagonist focused narrativism. The Z axis has got me perplexed however. Not in its concept, in fact I really get the whole Vx crazy sphere talk. Which takes me to my vision of Apoc Crazy...
Archetypes (or playbooks) are mapped on on 3d axes. Where they cluster for any given game is the potential for mindshere effectiveness. They represent the combined vision and priority of the group at play. Intrinsically the 'hollows' or empty spaces within the sphere(s) are places of little or no potentiial mindshare effectiveness. In fact they are places the players don't want to share. Unless.....
A canny MC embraces their role and lets the fiction decide something they genuinely care about whilst being a fan of the characters, the game will go to unexpected places whithin the sphere(s). New possibilities emerge and thus new playbooks are born. Faceless, Quarantine, Maestro 'D, The Horseman all explore a hitherto unplotted part of the sphere(s).
I'm guessing as we design playbooks, its almost from an MC perspective. This archetype, this plot point, allows me to explore new territory of our unique vision of apoc world. The beauty lies in the concious decision, in fact the fora are full of playbook designers stating exactly why they went down a certain path with their creations, justifying implicitly their X and Y axis, yet unambigiously declaring their desire for where they want the Z axis to go.
More later, I'm melting my own mind!