One possible plot could be that Superman arrives in the Naruto universe. Naruto, being curious about Superman's powers, challenges him to a friendly spar. As they fight, they realize they both share a strong sense of justice. Then they team up to fight a common enemy, like a powerful new villain who has the ability to steal chakra and weaken Superman with kryptonite - like substances found in the Naruto world.
In a 'naruto and superman crossover fanfiction', the plot could be that a powerful dark force has managed to create a rift between dimensions, pulling Superman into the Naruto universe and some Naruto characters into Superman's world. Superman has to adapt to the new world with its different rules of power like chakra. Meanwhile, Naruto and his friends in Superman's world have to deal with Lex Luthor - like villains. Eventually, they find a way to close the rift and return to their respective worlds, but not before they exchange knowledge about their powers and ways of life.
One idea could be that an alternate - universe version of Superman comes to their world. This could lead to a power struggle as both Supermen have different ideas on how to protect the city. Another plot could involve Lois getting superpowers temporarily and having to learn how to use them while also dealing with her regular reporting job.
Here's an idea. There could be a powerful magical artifact that surfaces in both cities. It has the power to change reality. Superman, being a man of science, is initially skeptical about its power. Batman, on the other hand, has seen enough strange things in Gotham to know it's dangerous. They have to overcome their differences in beliefs as they race against time to stop different factions in both cities who want to use the artifact for their own gain. Along the way, they also learn more about each other's worlds and form a stronger bond.
One idea could be that Superman and Invincible have to team up to stop an alien invasion that threatens both their worlds. The two heroes could initially clash due to their different methods but then learn from each other.
One idea could be that Superman gets transported to the Marvel universe during a freak interdimensional event. He teams up with the Avengers to fight a common enemy like Thanos, who has somehow obtained Kryptonite. Their combined powers and different strategies would make for an epic battle and an interesting exploration of how the two universes' heroes interact.
One possible plot could be that Naruto gains Superman - like powers in the Naruto universe. He could use his super strength to lift entire mountains, which would be a huge advantage in battles against powerful foes like Pain or Madara. His super speed could make him almost invisible during combat, allowing him to outmaneuver his enemies with ease. And his heat vision could be a new form of jutsu that he discovers, adding a unique twist to his fighting style.
One plot idea could be Superman being transported to the Star Wars universe. He might use his powers to help the Rebel Alliance. For example, his super strength could be used to move large objects during battles or his heat vision to take out enemy ships. Another idea is that he teams up with Jedi. He could learn about the Force and they could combine their powers to fight the Sith.
One idea could be that Superman is somehow transported to the X - Men's universe. He teams up with the X - Men to fight a common enemy like a powerful mutant who has the ability to nullify Superman's powers on a large scale. They have to figure out how to combine their skills and powers to overcome this threat.
One idea could be that Batman and Superman are forced to team up against a common enemy from another dimension. This enemy has the power to manipulate the minds of the citizens of Metropolis and Gotham, making them turn on their heroes. Batman uses his detective skills to find the source of the mind control, while Superman has to hold off the hordes of brainwashed civilians.
One idea could be that Superman and Wonder Woman have to team up to stop an ancient alien threat that is immune to their individual powers. They must combine their strength, speed, and wisdom to find a way to defeat it.
One idea could be that a powerful new villain emerges who has the ability to manipulate both Batman and Superman's minds. This forces them to fight against each other while the real enemy is pulling the strings. Another could be a situation where Gotham and Metropolis are in a territorial dispute, and Batman and Superman initially take sides based on their cities, but then realize they've been played.