Perhaps there could be a plot where Kakashi is under a genjutsu (illusion). In this fanfiction, an enemy ninja has cast a powerful genjutsu on him that makes him act out of character, including appearing to be unfaithful to Sakura. Sakura then has to figure out that it's a genjutsu and break it to save their relationship.
Another could be that in the fanfiction, an evil doppelganger of Kakashi is created who goes around causing trouble, including making it seem like he's cheating on Sakura. The real Kakashi then has to clear his name while also dealing with the chaos the doppelganger has caused.
One idea could be that it's a misunderstanding. Maybe Kakashi was seen in a compromising situation with another character, but in reality, he was on a secret mission related to protecting Sakura. For example, he was getting information from an informant who was acting overly friendly, and it was misconstrued as cheating.
In a fanfiction, Sakura could be kidnapped by a group of rogue ninjas who want to use her medical skills for their own evil purposes. Kakashi gets wind of it and embarks on a solo rescue mission. Along the way, he has to fight through various traps and enemies. When he finally reaches Sakura, he has to protect her from the final boss - like rogue ninja while also helping her regain her confidence after being held captive.
Another idea is that Sakura, feeling she still has much to learn from Kakashi, asks him for one - on - one training. As they train, Kakashi reveals some of his own past traumas which Sakura helps him deal with in her own way, while also growing stronger in her ninja skills. This could also lead to a deeper bond between them as teacher and student.
In a fanfic, it could be that Sakura is wrongly accused of cheating on Sasuke. Maybe a false rumor spreads in the village. The fanfic could then focus on how Sakura and Sasuke deal with this rumor. Sasuke might initially be angry but then realizes he should trust Sakura more. And Sakura has to fight against the judgment of the village while trying to prove her loyalty to Sasuke.
They could time - travel during a great war in the past. Kakashi and Sakura might find themselves in the middle of battles they only read about in history books. They could use their modern - day skills to change the outcome of certain battles. This would also have a huge impact on the present - day Naruto world as they know it.
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 that Kakashi notices other ninjas showing too much interest in Sakura during a mission. So, he starts subtly making it clear that she's under his protection. For example, he always pairs up with her during dangerous tasks and gives warning looks to those who get too close.
Well, perhaps the child in the 'kakashi and sakura child fanfiction' gets involved in a mysterious mission that only they can solve because of their special heritage. The child could be the key to uniting two feuding ninja clans, using the influence of Kakashi and Sakura in the ninja world. Also, the story could focus on the child's relationship with their peers, who may be in awe or jealous of their powerful parents.
One idea could be Sakura exploring her unique family dynamic with Kakashi as her father. Maybe she inherits his ninja skills in an unexpected way.
One idea could be a time - travel fanfiction. Naruto, Sasuke, Sakura, and Kakashi are sent back in time to their early days at the academy. They have the knowledge of their future selves and use it to change some of the events that led to hardships in their lives. For example, Sasuke could prevent the Uchiha massacre with their combined help.
A fanfiction where they all go on a mission together in a strange, uncharted land. Kakashi is the leader as usual. Sasuke could struggle with his inner demons during the mission, Naruto would be trying to keep everyone's spirits up, and Sakura would be using her medical skills to keep them all in top shape. There could be new enemies and challenges that they have to face together, forcing them to rely on each other more than ever.