Underground Ninja society?! I would prefer a barbarian mod, something about battling other wandering barbarian tribes and claiming territory sounds fun to me, although you guys may have a different view on how to build the mod entirely. Still, if you could get some small tribal battles to occur, certain tribes using certain spells as their trademark, and each tribe having unique strengths and weaknesses, that would be awesome. You'd need to know your scripting though.
It seems more people want ninjas. I like the idea of an underground society, but viewing 'ninjas' in the game world seems unoriginal to me. Just a suggestion since I'm not building the mod, I'd be more susceptable to playing it if the underground society had something unique and original about them, something that ninjas do not have.
I still think the barbarian mod sounds like a great idea, I'd go off of that one personally, but to each their own