Maybe an OC from a different village who is sent on a mission with Kakashi. Their different backgrounds and skills create an interesting dynamic during the mission. For example, the OC could be a master of water jutsu and Kakashi has to adapt his combat style to work with them.
An OC who is a medic - ninja and has to patch up Kakashi after a tough battle. Over time, a deep friendship or something more could develop between them as they spend more time together.
One idea could be an OC who is a new ninja in the village and Kakashi becomes their mentor. The fanfic could explore their training sessions and how the OC grows under Kakashi's guidance.
One idea could be a crossover with the Harry Potter universe. Kakashi could be a visiting wizard from another dimension, teaching the students at Hogwarts some of his ninja skills. Another could be a crossover with the Avengers. Kakashi's stealth and combat skills would make him a great addition to the team, and it would be interesting to see how he interacts with the likes of Iron Man and Captain America. A third idea might be a crossover with the Lord of the Rings. Kakashi could be a ranger in Middle - earth, using his Sharingan to detect the presence of orcs and other dark forces.
Well, a cool plot might be Kakashi time - traveling to the era of the First Hokage. There, he could learn ancient jutsu and bring them back to his time. Maybe he gets involved in a conflict that was long forgotten but has implications for his present. He could also time travel during a crucial battle in his own past, like the fight against Obito, and change the outcome in unexpected ways.
One idea could be an OC Hatake who has a unique chakra ability. For example, they can manipulate ice in a way that's different from Haku. They could use this to create ice sculptures that come to life in battle. Another plot could involve the OC Hatake being a long - lost relative of Kakashi. This would bring in a lot of family - related drama and secrets to uncover. A third idea is that the OC Hatake has a special bond with animals, like being able to communicate with wolves, which is rare in the Naruto universe.
A plot could revolve around a mission where Kakashi and Sakura are paired up alone. They end up in a strange, enchanted forest. Sakura discovers a new type of medicinal plant that has the power to enhance chakra control. Kakashi helps her study it, and in the process, they learn more about each other's dreams and fears. It could also be a story of their friendship growing deeper.
One plot idea could be a time - travel scenario. Sakura somehow gets sent back in time and meets a younger Kakashi. They start off as strangers but as they train together, a bond forms. Another could be a mission - gone - wrong situation where they are stranded in a dangerous place and have to rely on each other for survival, which deepens their relationship. And a third idea is a body - swap situation where they have to understand each other's lives and feelings better while in each other's bodies.
Then there's Hana, an OC who is a sensor - type ninja. In fanfiction, she helps Kakashi in tracking down enemies and they form a unique bond as they rely on each other's skills during missions.
One plot idea could be that the OC is a ninja from another village who has a secret relationship with Kakashi. When she becomes pregnant, she has to decide whether to stay in her village or try to be with Kakashi in Konoha. This could create a lot of tension and drama, especially with the political implications between the two villages.
One idea could be that the OC, now as Kakashi, has to deal with the same traumas Kakashi had but with a different perspective from their original self. For example, they might handle the loss of their comrades in a new way that changes the course of events in the Naruto world.
Another possible plot is that Kakashi's adoption of Naruto draws the attention of some hidden enemies. These enemies might see Naruto as a threat because of his potential and Kakashi's influence. So they try to target Naruto, which leads to a series of intense battles and adventures where Kakashi has to protect Naruto and help him grow stronger to face these threats.