A new character could be introduced as the antagonist. Maybe someone who has also time - traveled and has their own agenda for the past. This character could be trying to preserve the original timeline for some reason, or they could be trying to create an even worse future. They would oppose Obito and Kakashi at every turn, creating a lot of conflict and drama in the fanfiction.
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 great 'Naruto x Obito Time Travel Fanfiction' is 'A New Chance'. It shows Obito traveling back in time and trying to change the course of events in the Naruto world. He teams up with Naruto at a younger age and they start to rewrite history, making some really interesting changes to the story we know.
Their relationships would become much stronger. Kakashi would no longer have to bear the guilt of Obito's 'death' and Rin's loss. Obito would be extremely grateful to Kakashi for saving his life and would likely view him as an even greater friend and ally. Rin would also be happy to have both of them by her side, and they could form a very tight - knit team.
There's also 'Time - Traveling Obito: A New Path'. This fanfic takes Obito back to his childhood days. He then uses his knowledge of the future to avoid the dark path he initially took. It delves deep into his character development and how he might have turned out differently. It's really interesting to see how he interacts with his younger self and the changes he makes to the timeline.
Most popular ones often focus on their deep emotional bond. Since they have a long history together in the Naruto series, fanfics tend to explore the different phases of their relationship, like their childhood friendship and how it was affected by the events that followed.
One common theme is redemption. Given Obito's past actions, many fanfics explore his journey towards redemption in the context of his relationship with Kakashi. Another theme is friendship turned into something deeper, as they have a long - standing connection that can be developed in various romantic or deeply emotional ways.
In a Sakura and Kakashi time travel fanfiction, they could end up in the past during a major war. Kakashi would be a great asset with his Sharingan and combat skills. Sakura, on the other hand, could help the wounded soldiers. Their presence in the past could change the outcome of the war. For example, they might be able to save some important characters who were supposed to die. This would then lead to a completely different future when they return to their own time.
Another theme is self - discovery. When they travel through time, both Kakashi and Naruto often learn new things about themselves. In some fanfics, Kakashi might come to terms with his past traumas in a different way. Naruto could also discover new aspects of his power. For instance, he might learn how to control his chakra better earlier in his life. This theme helps to develop their characters more deeply in the context of the time - travel story.