This would be rather a challenge but i think it can be done, and produce alot of fun in singleplayer.
The customizable SNPC would be common npc based on barneys code spawned using STOOL, but with some additional features:
-Selectable model (You can set any valid biped ragdoll for this)
-Changeable health and proficiency
-and most important setable behaviour against other npcs.
My idea is that this SNPC would have additional variable containing number of a team (0,1,2,3) and in STOOL options You could set relationship between that teams. I’ve suggested barney as a base because it would cut the basic problem of hostile NPCs not using npc weapons properly. That is it for now. How do You like it?