Maybe Naruto gets transported to a different realm where the Death God Slayer concept exists. There, he teams up with some local warriors who are also fighting against the Death God's tyranny. They teach Naruto their unique skills, and in return, Naruto shows them his ninja techniques. Together, they plan a big battle to defeat the Death God once and for all.
One possible plot could be Naruto discovers a hidden jutsu that allows him to combat the powers of the Death God. He trains hard to master it and then uses it to save his village from a new, powerful enemy who has the ability to control the Death God's powers. Along the way, he has to face his own inner demons and the doubts of his friends.
Perhaps in the fanfiction, the God of Death has been observing Naruto's journey all along. When Naruto is on the verge of losing a crucial battle against an overwhelming foe, the God of Death appears and offers him a pact. If Naruto can endure a set of challenges that test his will, his chakra control, and his understanding of life and death, he will be granted the God of Death's powers to turn the tides of the battle.
Perhaps an ancient evil that has been sealed away for centuries but has now found a way to use the concept of the Death God to break free. This entity was once a powerful ninja who turned to dark magic and now wants revenge on the modern ninja world. It could be a very powerful foe for Naruto as it has knowledge of old and forgotten jutsus.
A plot could involve a Fire God Slayer from another dimension accidentally ending up in the Naruto world. This character then has to find a way back home while also getting involved in the local politics and power struggles. They might use their Fire God Slayer powers to help the oppressed in the ninja villages, creating both allies and enemies along the way.
One idea could be a story where the protagonist, inspired by the Lightning God Slayer concept, develops a new jutsu that can counter the most powerful lightning - based attacks in the Naruto universe. This new jutsu could be a combination of wind and water elements to short - circuit the lightning.
Maybe Naruto already has some hidden ice - related abilities in his Uzumaki heritage that he didn't know about. In the fanfiction, he meets a group of Ice Demon Slayers who recognize this potential in him. They train him to master his ice powers so he can help them defeat an ancient ice demon that has awakened. Along the way, Naruto also teaches the demon slayers some of his ninja skills, creating a great exchange of abilities.
Perhaps Naruto enters the Death Parade world in search of a lost comrade. He teams up with the arbiters, reluctantly at first, when he realizes that they have some knowledge about where his friend might be. Along the way, he shows them the power of friendship and loyalty that he has learned in the Naruto universe, and they in turn show him the complex nature of the afterlife in the Death Parade world.
One possible plot could be that Naruto discovers a strange portal during one of his adventures. He steps through it and finds himself in the Pokemon world. Using his ninja skills and god - like powers (maybe something he acquired in his own world), he befriends Pikachu and they start a journey to stop an evil organization that is misusing Pokemon powers. Naruto's jutsu combined with Pikachu's electricity attacks would make for some epic battles.
One possible plot could be Naruto being the son of a Greek god in secret. He discovers his divine heritage during a mission in the Naruto world. With his newfound godly powers, he can control elements in a way no one else can, like summoning lightning bolts similar to Zeus or controlling water like Poseidon. This would completely change the power dynamics in the Naruto universe.
One idea could be that the main character, who has the power of a Sky God Slayer, teams up with Naruto to fight a new, powerful enemy that threatens the ninja world. This enemy could be manipulating the weather in a destructive way.
One way Naruto could learn God Slayer magic in fanfiction is through a magical artifact. He might stumble upon an ancient relic during one of his missions that imparts the knowledge of God Slayer magic. Another possibility is that he meets a powerful mage who decides to teach him the magic in exchange for a favor, like helping to protect the mage's homeland.