One idea could be that Obito and Kakashi are sent back to their childhood days. Obito then tries to prevent certain tragedies that happened in their past, like the death of his comrades. He might also try to change Kakashi's cold - hearted nature that developed over time. This could lead to a whole new set of relationships and events in the Naruto world.
One idea could be that Kakashi travels back to the moment right before Obito's 'death'. He uses his knowledge of future events to prevent the cave from collapsing on Obito. Then, together they are able to rescue Rin more effectively and change the whole tragic course of events.
One common theme is redemption. Since they both have a lot of regrets in their original timeline, time travel gives them a chance to fix things, like saving their friends or preventing the destruction of the village.
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, 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.
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.
Maybe the OC is a time - traveler who accidentally lands in Obito's time. She knows about his future actions but instead of trying to stop him right away, she tries to understand his pain. This leads to a complex relationship where Obito is both intrigued and afraid of her knowledge.
One idea could be a story where Naruto somehow travels back in time and meets a younger Obito. He tries to change Obito's path before he turns to the dark side. This could involve Naruto showing Obito the true meaning of friendship and the value of the Hidden Leaf village.
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.