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.
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.
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.
Research the Naruto universe thoroughly. Know the rules, the other characters, and the overall atmosphere. When writing about Kakashi, stay true to his character. He's calm, intelligent, and has his own set of traumas. Incorporate your OC into this world in a believable way. You could also add some plot twists to keep the readers engaged, like the OC having a secret connection to an old enemy of Kakashi.
One common theme is mutual understanding. Since they both have complex pasts, fanfictions often explore how they come to understand each other's pain and experiences. For example, Kakashi's loss of his comrades and Anko's past with Orochimaru.
I really like 'A New Bond'. In this fanfiction, Kakashi and Naruto go on a mission together that is not part of the original storyline. This allows for a lot of character development between them. It's full of action and also some really heart - warming moments.
In this fanfic, Obito's character development could be quite interesting. He starts as a young, eager ninja with big dreams. But after his experiences, he may become more jaded and vengeful. Kakashi, on the other hand, may develop from being a cold - ish ninja to someone more understanding and empathetic towards Obito as the story unfolds in the fanfic.
The relationship in 'Obito Uchiha x Kakashi Hatake Fanfic' can be seen as a study of contrast and connection. Their bond was initially based on being comrades. However, as the story progresses in the fanfic, it could show how their divergent paths, like Obito's fall to the dark side, affected their relationship. It might also explore themes of forgiveness and redemption as Kakashi may have to face his old friend who has now become an enemy.
There's also 'Under the Cherry Blossoms'. In this one, Sakura is on a mission with Kakashi, and during their journey, they get stranded in a forest. While waiting for rescue, they have long conversations about their pasts, their dreams, and slowly realize that they have more in common than they thought. It's a beautifully written story that really delves into their characters.