I'm not sure specifically as there could be many different plots for such a fanfic. It might involve Naruto characters being transported to the X - Men world or vice versa, and then they have to deal with the different powers, cultures and challenges in that new world.
The plot could involve Naruto characters being transported to the X - Men world or vice versa. They might team up to face a common threat that is affecting both worlds, like a powerful villain who is stealing chakra and mutant powers. Maybe Naruto and Wolverine could bond over their shared experiences of having a power within them that they have to constantly control.
Definitely Naruto from the Naruto series. And from the X - Men, characters like Wolverine or Professor X are very likely to be main characters as they are some of the most well - known X - Men.
I think Naruto and Rogue would be an interesting pairing. Naruto has a strong will and positive energy, and Rogue is always worried about her power harming others. Naruto could perhaps help Rogue gain more confidence in herself and her abilities. Also, his chakra might interact in an unexpected way with Rogue's power - absorption ability.
In a Ben 10 and Naruto fanfic, their powers would be a great combination. Ben has the ability to transform into various alien forms, each with unique powers. Naruto has his chakra and his powerful jutsu. They could learn from each other. For example, Ben could teach Naruto about some of the alien technologies, while Naruto could teach Ben about chakra control and the ninja way.
Well, you could start by having a common villain. For example, the Green Goblin could team up with a mutant - hating group. This would force Spiderman and the X - Men to work together. Spiderman might be initially against the X - Men because he's not used to their way of doing things, but as they face the common threat, they start to understand each other.
One idea could be that Ben Tennyson, while on a mission in another dimension, accidentally lands in the X - Men's universe. His Omnitrix starts acting strangely due to the different energy signatures there. The X - Men initially see him as a threat but then realize he could be an ally in fighting a common enemy, like a powerful mutant - alien hybrid that is causing chaos. Ben could transform into some of his alien forms to help the X - Men in battles, and there could be interesting interactions between the different characters, like Gwen and Rogue comparing their powers.
One idea could be that Spiderman gets caught in the middle of a conflict between the X - Men and one of their powerful enemies. Maybe Magneto is causing chaos in New York and Spiderman tries to stop him on his own at first, but then realizes he needs the X - Men's help. The X - Men, in turn, are initially skeptical of Spiderman but come to respect his skills.
The values of the two worlds could also merge. In Naruto's world, the concept of protecting the village and comrades is very important. In One Piece, it's about the crew and the pursuit of dreams. Characters from both worlds could learn from each other. For instance, the Straw Hat crew could learn about the loyalty within a ninja village, and the ninjas could understand the freedom - loving spirit of the pirates. This would lead to new character developments and story arcs in the fanfic.
One interesting crossover could be Naruto learning some of the X - Men's powers. For example, he might gain the ability to control magnetism like Magneto and use it in combination with his own chakra control for unique attacks.