Perhaps there could be a fanfiction where a new character is introduced who has a power that accidentally creates a situation similar to the relationships in 'Kiss x Sis' within the Naruto universe. This new character's power causes some confusion and hilarity as the Naruto characters try to figure out how to deal with these unorthodox relationships while still maintaining their ninja duties.
They could participate in a school competition together, like a sports event or a talent show. Keita and Riko would practice a lot, and during this time, they would have many sweet and funny moments. There could be some friendly rivalry with other students too. And as they strive for victory, their relationship could develop further, with Riko cheering Keita on in a really enthusiastic way and Keita being inspired by her support.
One idea could be creating an OC who is a new student at their school. This OC could be initially oblivious to the unique relationship between the twins. Then, as the story progresses, the OC gets drawn into their world, perhaps helping them deal with the challenges of their taboo relationship in a discreet way.
A third idea could be an OC who is an artist. The OC could be hired to paint a portrait of the sisters. During the process, the OC gets to know them on a deeper level. The sisters are intrigued by the OC's creative process and start to develop feelings for the OC. This could lead to a complex love triangle - like situation where the OC has to navigate their feelings for the two sisters while also dealing with their own artistic goals.
One interesting plot idea could be Keita and Yuzuki getting lost in a magical forest together. As they try to find their way out, they rely on each other and their relationship deepens. Another could be them having to work on a school project together where they face many obstacles but learn more about each other's strengths and weaknesses in the process.
One plot idea could be that Keita struggles in a particular subject at school and Sensei offers him extra tutoring. During these tutoring sessions, they gradually develop a deeper understanding of each other and form a unique bond. This could lead to some comical moments as Keita's sisters might get jealous of the time he spends with Sensei.
I'm not entirely sure as the 'kiss x sis' series contains some inappropriate content. But generally, in anime, an ending story often ties up loose ends, shows character growth, and concludes the main story arcs.
A plot idea could be that Nanami accidentally loses her memories and Tomoe has to help her regain them. He would take her to all the places that are important to their relationship, like the shrine where they first met. Along the way, Tomoe would be patient and kind, and as Nanami slowly starts to remember, their bond would become even stronger.
One idea could be that the reader is a new shrine maiden and Mizuki has to teach them the ropes. They could start off not getting along because the reader is stubborn and wants to do things their own way, but over time they develop a deep bond as they face various challenges at the shrine 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.