Perhaps the Guyver is a relic from an ancient civilization in the Naruto world that was long forgotten. Naruto stumbles upon it during his training journey. As he bonds with the Guyver, it unlocks memories of a past war where the Guyver was used against a great evil. Now, Naruto has to use the Guyver's powers to face a new threat that is emerging in the ninja world, something that is related to the same evil from the past. He also has to deal with the political implications as some ninja villages fear his newfound power.
Definitely Naruto and the main Guyver character. Naruto because of his determination and growth throughout the Naruto series. He would be really interesting to see how he adapts to the Guyver's powers. And the Guyver, because its powers are so unique and different from the ninja abilities in Naruto. Their interactions could be full of misunderstandings at first as they come from different worlds, but then they could learn from each other.
One possible plot could be that Naruto has to go on a special mission with Kurenai. They might be sent to a hidden village where they discover a new type of jutsu that is being misused. Kurenai's genjutsu skills and Naruto's rasengan could be combined in unique ways to fight against the villains using this jutsu.
One possible plot could be that Riddick somehow gets transported to the Naruto universe. He could initially be seen as a threat, but then he teams up with Naruto and the others to fight a common enemy, like a new powerful Akatsuki member who has powers similar to some of the creatures in Riddick's world. Riddick's combat skills and his ability to see in the dark could give the Naruto gang an edge in battles.
One interesting plot could be that Naruto and The Flash team up to stop a villain who can manipulate time. The Flash uses his speed to move between different time periods while Naruto uses his shadow clones to gather information across a large area. They would have to learn to combine their powers and trust each other's instincts in order to succeed.
Well, perhaps in this crossover, Sephiroth is a rogue ninja from a far - off land that has powers similar to his in the Final Fantasy world. He starts causing destruction in the Hidden Leaf Village. Naruto, being the hero, decides to face him. Along the way, they discover that their powers have some strange similarities. For example, Sephiroth's sword techniques could be compared to some of the advanced sword - using ninja in the Naruto world. And they might end up having to team up against a greater evil that emerges because of Sephiroth's arrival.
One possible plot could be that Lucifer, bored with the underworld, decides to visit the Naruto universe. He could take on the form of a powerful ninja and start causing chaos, which draws the attention of Naruto and his friends. Naruto, seeing the evil in Lucifer's actions, tries to convince him to change his ways, leading to a series of battles and philosophical debates.
A cool plot could be that there is a prophecy in both the Naruto and Percy Jackson worlds. It predicts the coming together of a shinobi and a demigod to save both realms from a powerful, interdimensional evil. Percy arrives in the Naruto world, and he and Naruto have to learn each other's skills. Percy teaches Naruto about Greek mythology and how to fight mythical creatures, while Naruto shows Percy the ways of chakra and ninjutsu. They then journey to different parts of the Naruto world, gathering allies and knowledge to face the impending doom.
One possible plot could be that Naruto, while on a strange mission, gets transported to the Witcher's world. He teams up with Geralt to hunt a powerful demon that has the ability to control minds. Naruto's unique ninja skills, like his shadow clones, would be very useful in distracting the demon while Geralt figures out its weakness.
In a Naruto and Bakugan crossover, it could start with a strange portal opening in Konoha. Out of it come some of the Bakugan and their brawlers. At first, there's confusion and some tension between the two groups. But then they realize that a dark force is behind the portal's appearance and it plans to use the combined powers of chakra and Bakugan energy for evil. Naruto, being the hero he is, steps up to lead the combined group. They travel to different locations in both worlds to gather resources and allies. Along the way, Naruto and the Bakugan brawlers bond over their shared determination to protect their homes and loved ones.
Maybe the Naruto characters are intrigued by the beyblades. Naruto himself could see the beyblades as a new form of training. He might challenge his friends to beyblade battles to improve their reflexes and teamwork. Meanwhile, the beyblade characters could learn about chakra and ninjutsu from the Naruto crew. This exchange of skills could lead to some really unique battles where both beyblade and ninja powers are used in creative ways.