One possible plot could be that Kushina somehow gains the powers of the Gura Gura no Mi. She could use it to protect Naruto from various threats in the Naruto universe. Maybe she could create powerful shockwaves to fend off enemies like the Akatsuki. Naruto, seeing his mother's new power, would be inspired to train harder and learn how to combine his own jutsu with the power of the Gura Gura no Mi.
In a fanfiction about Naruto, the Gura Gura Fruit, and the Third Shinobi War, perhaps the Gura Gura Fruit is a new, mysterious element introduced into the war. Naruto, being the curious and adventurous ninja he is, might be the first to discover it. His friends and allies could be initially wary of this strange power, but as Naruto learns to control it, they see its potential in turning the tide of the war against powerful enemies such as Orochimaru or the Hidden Stone Village's forces.
Their relationship could become stronger. Kushina, with the Gura Gura no Mi, would be an even more powerful protector. Naruto would look up to her even more, and they would bond over the new power she has. He might ask her for training tips on how to use such a powerful ability.
One possible plot could be Naruto somehow obtaining the Gura Gura Fruit. He then has to learn to control its powers while still maintaining his own ninja skills. For example, he might accidentally cause earthquakes during battles at first, but as he masters it, he combines the fruit's power with his Rasengan, creating a super - powerful attack.
In this fanfiction, Naruto with the Gura Gura Fruit might face a new type of antagonist. This antagonist could be someone who is also after the power of the fruit or someone who has a power that can counteract Naruto's new abilities. Naruto would have to figure out how to use the Gura Gura Fruit in combination with his existing ninja skills, like his shadow clones. He could create clones and then use the fruit's power through them to launch multiple attacks simultaneously, which would be a very interesting development in the story.
Naruto could use the Gura Gura Fruit to create huge tidal waves in battles near water sources. This would be a great advantage against enemies not used to fighting in water. For example, if facing the Sand Village ninjas, he could flood the desert battlefield, disrupting their attacks and movements.
Naruto would become more powerful, and this might make him a bit more cautious. He's used to his regular ninja abilities, but the gura gura fruit's power is on a different level. He might be more hesitant in battles at first, not wanting to accidentally cause too much destruction with the fruit's earthquake - causing ability.
His self - confidence might change. On one hand, having this new powerful ability from the Gura Gura Fruit would boost his confidence. But on the other hand, he would also realize that he has a huge responsibility with these powers. He might become more introspective, thinking about how to use the power for good and not let it corrupt him, like how he was always careful with the power of the Nine - Tails inside him.
One idea could be a crossover with the One Piece world and the Fairy Tail world. The ope ope no mi user could use their powers to heal the mages in Fairy Tail who get injured during battles. Another idea is a crossover with the Naruto universe. Maybe the ope ope no mi could be used to perform unique surgeries on ninja who have suffered severe injuries from jutsus. A third idea is a crossover with the Harry Potter world. The ope ope no mi user could use their powers to reverse some of the dark magic effects on students at Hogwarts.
To start writing an ope ope no mi crossover fanfiction, you should begin by brainstorming the crossover concept thoroughly. Consider the rules and logic of both the One Piece world (where the ope ope no mi exists) and the other universe you're crossing it with. If it's a crossover with the Marvel Universe, for instance, think about how the ope ope no mi's healing and operating abilities would fare against superpowers like Wolverine's regeneration. Outline your story, including the introduction of characters, the conflict, and the resolution. You can start with a scene that showcases the uniqueness of the ope ope no mi in the new crossover setting, perhaps a scene where the user has to use their power in a completely different way than in One Piece.