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.
A great plot idea for Kakashi Hatake time travel fanfiction is that he travels back to his childhood days. He could use his knowledge and skills from the future to help himself grow stronger faster. He might also be able to prevent some of the tragedies that happened in his youth, like the loss of his comrades at an early age. He could interact with his younger self and give valuable advice, which would change the way he developed as a ninja.
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.
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.
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.
One interesting plot idea could be Kakashi traveling back in time to before the Uchiha massacre. He could try to prevent it, which would completely change the future of the Naruto world. Maybe he'd have to face off against Danzo and his schemes early on.
A cool plot would be Kakashi traveling back to a time during one of the great ninja wars, say the Third Great Ninja War. He could use his knowledge of future jutsu and strategies to turn the tides of battles that were previously lost. He might also be able to save comrades who died in the original timeline, and in the process, change the future of many ninja families.
Well, in some 'kakashi time travel fanfiction', Kakashi might travel back to change the fate of his village. He could go back to a time when there were internal conflicts in Konoha and resolve them before they escalated. Also, he could have the chance to reunite with his loved ones who had passed away, like his father. This would add a whole new layer of emotional depth to the story.
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.
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.
One plot idea could be that fem Naruto and Kakashi travel back to the founding of Konoha. They could use their modern - day knowledge and skills to influence the early development of the village in positive ways, like introducing new training methods. Another idea is that they travel to a time when a great enemy first emerged and they work to stop the threat before it becomes too powerful.