One interesting plot could be Naruto being transported to the world of Diablo 3. He might initially be confused by the dark and demonic environment but quickly adapt his ninja skills. For example, his shadow clone jutsu could be used in unique ways, like sending clones to explore different areas filled with demons while he focuses on a main objective. His chakra could also be seen as a form of magic in the Diablo 3 world, and he could learn to combine it with Diablo - style spells or abilities.
In a Naruto x Diablo 3 fanfiction, the plot could involve Naruto's search for a way back to his own world. He gets caught up in a conflict between the angels and demons in Diablo 3. He discovers that some of the demonic powers are similar to the dark chakra he has encountered in his own world. Naruto decides to use this knowledge to his advantage. He tries to find a way to use the energy in Diablo 3 to open a portal back home while also helping the good - sided characters fight off the demon hordes.
One interesting crossover could be Naruto characters in the Diablo world facing Diablo's demons. For example, Naruto using his shadow clones to fight hordes of imps. Another could be Sasuke with his Sharingan trying to decipher the magic in Diablo's world. It would be really cool to see how Naruto's chakra system would interact with Diablo's magic system.
Mephisto could also be a great main antagonist. His ability to corrupt and manipulate could pose a real threat to Naruto. Mephisto might try to turn Naruto's friends against him or corrupt Naruto himself. Since Naruto values his comrades highly, this would be a difficult situation for him to deal with. And if Mephisto manages to get into Naruto's head, it could lead to some really intense and dark moments in the fanfiction.
One possible plot could be that Naruto is somehow transported to the X - Men universe. He teams up with Wolverine. Naruto's unique ninja skills, like his shadow clones, could be used in combat against the Brotherhood of Mutants. Wolverine could teach Naruto about the concept of mutants and their struggles, and Naruto could in turn show Wolverine some of his ninja wisdom.
One interesting plot could be that Naruto is somehow transported to the X - Men universe. He teams up with Wolverine. Naruto's ability to create shadow clones could be used in interesting ways during battles, like confusing the enemies. Wolverine could teach Naruto some combat skills from the X - Men's fighting style. They could go on a mission to stop a powerful mutant villain who has plans to take over the world, using Naruto's chakra powers and Wolverine's adamantium claws.
Well, a plot could involve a villain in the Naruto world who is creating some sort of dark energy that is affecting both dimensions. Kirby, being a hero in his own world, senses this and travels to the Naruto universe to stop it. Along the way, he teams up with Naruto. Naruto is amazed at Kirby's power to transform by swallowing enemies. Together, they face off against the villain and his minions, with Kirby using his copied ninja skills and Naruto using his jutsu.
One possible plot could be that Robocop is somehow transported to the Naruto universe. He teams up with Naruto to fight against a new threat that is using advanced technology similar to what created Robocop, but in a more malicious way. Naruto's ninja skills combined with Robocop's firepower would make for some epic battles.
One possible plot could be that Naruto is on a mission in a desert area and accidentally stumbles upon the tomb of the mummy. When the mummy awakens, Naruto initially tries to fight it, but then realizes that the mummy has some sort of connection to an ancient chakra source. They then form an uneasy alliance to explore and protect this power from falling into the wrong hands.
One possible plot could be that Naruto somehow gets transported to the Titanfall universe. He might use his ninja skills to outmaneuver the Titans at first, but then he starts to learn about the technology there and teams up with some of the Pilots. Maybe he teaches them some of his chakra - based techniques in exchange for learning how to pilot a Titan.
In a 'Naruto x Wanda fanfiction', it could start with Naruto on a strange mission that accidentally leads him to Wanda's dimension. Wanda is in the middle of a battle with her own enemies. Naruto jumps right in to help. After the battle, they realize that they have much to learn from each other. Wanda can teach Naruto about different forms of magic, while Naruto can show Wanda his ninja techniques. They then form an alliance to protect both their dimensions from future threats.
One possible plot could be that the Terminators are sent to the Naruto universe to eliminate key ninja. Naruto, being Naruto, would not stand for this and would start fighting back. He could use his shadow clones to confuse the Terminators and then use Rasengan to try and damage them.