One possible plot could be that Naruto discovers a strange gun in a hidden dungeon during one of his missions. He initially has no idea how to use it but as he studies it, he realizes it has some unique powers. Maybe it can shoot chakra - based bullets that can immobilize enemies stronger than him, which changes the way he approaches battles.
Well, perhaps the gun is a gift from a mysterious figure who claims to be from a far - off land. Naruto gets the gun and has to keep it a secret from his friends in Konoha at first. He then starts training with it on his own, learning how to aim and fire it accurately. As he gets better, he uses it to defend the village from a new kind of threat that normal jutsu can't handle easily.