Another plot could be a battle scenario. Android 21, with her combat abilities, gets into a fight with Naruto. Naruto initially struggles because he's not used to her style of fighting. But as the battle progresses, he starts to figure out her weaknesses and uses his ninja skills like shadow clones to outmaneuver her. In the end, they might respect each other's strength and start to cooperate.
One idea could be that Android 21's unique powers attract the attention of Naruto. He might see her as a new kind of challenge. Maybe she has some sort of energy that is similar to chakra in a way. And Naruto tries to understand her and her powers, leading to an unexpected friendship.
Maybe in the fanfiction, Android 18 gets injured while protecting Gohan from a new powerful enemy. Gohan then takes care of her and in that time of vulnerability, they realize their feelings for each other. This could lead to them exploring a relationship while also trying to figure out how to defeat the enemy that still lurks.
One idea could be that the OC is a scientist who is intrigued by Android 18's unique blend of power and humanity. He tries to find a way to make her more 'human' in terms of emotions, leading to a series of adventures where they face various threats together. Another could be that the OC is a time - traveler who accidentally lands in the Dragon Ball universe and forms an alliance with Android 18 to prevent a catastrophic event in the future. And a third idea might be that the OC is a martial artist who challenges Android 18 to a series of battles, which gradually turn into a friendship and then something more romantic as they learn about each other's values.
One idea could be Android 21's struggle to fit into a world that fears her. She has to prove that she's not a threat and tries to make friends. Another could be her discovery of new powers and how she uses them for good. Maybe she teams up with other characters to fight a common enemy.
One idea could be that Goku and Android 21 team up to face a new and extremely powerful villain. This villain could be from another dimension and has the power to corrupt the minds of other fighters. Goku, with his pure heart and Android 21's intelligence, find a way to defeat the villain.
One interesting idea could be a battle scenario where Naruto's ninja skills are pitted against Android 18's superhuman strength. Maybe they start off as opponents but then have to team up to face a greater threat, like an ancient evil that has emerged in both their worlds. Another idea could be a story where Android 18 ends up in the Naruto universe and has to adapt to the ninja way of life. She could learn chakra control and use it in combination with her android powers.
One idea could be a story where Gohan and Android 18 team up to protect the Earth from a new, powerful threat. They start off as reluctant allies but gradually develop a deep respect for each other's abilities. Another plot could be a time - travel story where they go back in time and have to prevent an event that changes their future relationship. Maybe they end up having to work together to stop a younger version of Cell from evolving further.
A great plot idea would be Android 21 crossing into a world where technology is feared. She'd have to prove that not all advanced technology is bad. Also, she could cross into a world of pirates, using her skills to navigate the high seas and fight off other pirates.
One plot idea could be that Ash and Android 21 team up to fight a common enemy in a tournament. Another could be a story where Android 21 tries to understand human emotions through Ash's experiences. Or perhaps a plot where they travel through different dimensions together.
A plot could revolve around Sesshomaru being intrigued by Naruto's determination. Sesshomaru decides to train Naruto in some of his own combat techniques, which Naruto adapts to his own fighting style. In return, Naruto shows Sesshomaru the value of friendship and loyalty within a village. This could lead to Sesshomaru having a change of heart about his own solitary ways. Also, they could team up to face a common foe from Sesshomaru's world that has somehow entered the Naruto universe.
One idea could be that Naruto and Kashuina meet during a mission outside of Konoha. They start off as rivals, constantly trying to outdo each other in combat skills. But as they face various challenges together, they gradually develop a mutual respect and then deeper feelings. Another plot could be set in a post - war scenario where Naruto, now a hero, meets Kashuina who has a unique ability to heal the psychological scars of the war - torn ninjas. Their interaction then leads to a romantic relationship. And a third idea is that they are both students under a new sensei who pairs them up for special training. During this training, they discover things about each other that they never knew before and fall in love.