When I created the full edition of Zombies Attack (my zombie survival game, you have downloaded and played it right? I mean its an off shoot of Rogue Space game design so don't fear the dead) I came up with a simple mechanic for NPC's. Instead of being fully created with all attributes, only certain critical attributes were listed with a bonus.
This gives the referee an idea of focus for the foe in question and in play where/how the npc will act. The more attributes listed, the more dynamic, and potentially deadly, the foe. If an attribute is not listed? You can still use it, but there is no bonus for the creature/NPC using that attribute. Simple clean and it does not require a wonky sub-system to make them work. Notes on damage, size, etc are still used/useful but the base game mechanics are more streamlined and fluid.
Before I go mucking about and potentially "fixing" something you do not believe is broken, please let me know what you think. Time is short as all the art work is complete and the text is being redone. I think rather than adding an adventure I may add a section on bionics and mutations. Those might be fun and more inspiring. As always your feedback (even the meh for my other efforts) is welcome.