Perhaps Naruto is reincarnated as the Soul King. In this plot, he has to deal with the internal politics of the Soul King's domain. He could try to bring his ninja values of friendship and hard work to reform the way things are done there. And at the same time, he has to fight off enemies who want to take over the power of the Soul King.
The plot could start with Naruto already being the Soul King but having amnesia. As he slowly regains his memories, he realizes his true power and purpose. He then unites different factions in the Soul King's world using his charisma and leadership skills that he developed in the ninja world. Along the way, he discovers new abilities related to the Soul King and uses them to protect the world from an ancient evil that has awakened.
One possible plot could be that Naruto is somehow transported to the world where the Soul King exists. He discovers his new role as the Soul King and has to learn the rules and powers associated with it. He might use his ninja training to face the threats that come to the Soul King's realm.
Well, it might be about Naruto being in the Soul Society world and being neglected. Maybe he's not given proper training or recognition because he's an outsider. He could be struggling to prove himself among the Soul Reapers.
One idea could be that Naruto discovers he has a hidden connection to the Soul King. For example, his chakra is somehow linked to the power of the Soul King. This could lead to new abilities and a whole new arc in the story where he has to explore this connection and face enemies who want to use it for their own evil purposes.
Madara Uchiha could also be a great antagonist. Imagine if he came back and thought that the power of the Soul King could be used to achieve his ultimate goal of creating a world under his control. He'd be a formidable foe for Naruto, especially since he has so much experience and power already.
It could be about Naruto discovering a hidden forest realm where he has to prove himself as the king. Maybe he has to face off against ancient forest spirits or creatures to earn that title.
Well, it could be about Naruto discovering a hidden dragon realm. He might learn to communicate with dragons and gain new powers from them. Maybe he has to fight against an evil force that wants to control the dragons for its own dark purposes.
Maybe Naruto discovers he has a hidden connection to a powerful demon king. This could lead to him having to fight against those who fear his new - found power, while also trying to control the dark forces within him.
Well, it could be about Naruto gaining the power to control the King of Hell. Maybe he makes a deal with it to save his friends. For example, he might offer his own life force in exchange for bringing Sasuke back from the brink of darkness.
Well, it could be about Naruto achieving an even higher level of power and becoming the true 'king of kings' in the ninja world. Maybe he unites all the hidden villages under his leadership.
Well, it could be that Naruto somehow gets transported to the world where King Hassan exists. Maybe Naruto, with his ninja skills and determination, is mistaken for a great warrior like King Hassan. He could then have to take on the role and face the challenges that King Hassan would face, like protecting a certain realm or fighting powerful enemies using a combination of his ninja jutsus and the powers associated with King Hassan.
One possible plot could be that Naruto somehow gets transported to the world of the Witch King. He might initially struggle to understand the dark magic and new rules there. But being Naruto, he quickly adapts and uses his ninja skills in combination with the newfound magic. Maybe he tries to fight against the tyranny of the Witch King's regime from within, using his charm and determination to win over some of the locals who were forced to serve the Witch King.