Hooper_X man, a Ninja-themed Left 4 Dead mod would be pretty fucking great - the Survivor Team are samurai (start out with swords but can upgrade to that big long polearm thing or a bow & arrow or whatever) and they have to fight off hordes of shitty regular ninjas (the zombies) as well as special Ninja Masters with their own ridiculous weapons. Smoker has one of those Scorpion-from-mortal-kombat deals, the Boomer is basically this guy, etc.
Just swap out all the grunting and groaning for high-pitched KEE YAHS! and you're on to something.
(Alternately, leave the survivors TOTALLY UNCHANGED.)