One possible interesting plot could be about changing a historical event and dealing with the unforeseen consequences. For example, a character might go back in time to prevent a war, but then realizes that this action led to other negative outcomes like economic collapse.
Another plot could be that they travel to a future time where music has evolved into something completely different. They struggle to understand the new forms of music but also bring back some of the old charm from their time, creating a unique blend that revolutionizes the future music scene.
One interesting plot could be traveling back in time to the era when the Portal was first created. The protagonist could try to prevent certain events that led to the creation of the dangerous test chambers. For example, they might stop the initial, unethical experiments on sentient beings. This would change the whole course of the Portal universe and create a new future.
One interesting plot could be Sarada traveling back to the time of the founding of Konoha. She might witness the great battles and negotiations that led to the creation of the village. This would give her a new perspective on the history and values of her home.
One interesting plot could be that Naruto and Yoshina time travel to the era of the Sage of Six Paths. They could learn about the origin of chakra directly from the source. Naruto might gain new powers or a deeper understanding of his own abilities. Yoshina could also have her own revelations. Another plot could be that they are sent back to when Naruto was a child and they have to prevent Orochimaru from infiltrating the village earlier. This would change the power dynamics in the village and Naruto's growth path.
One interesting plot could be traveling back to the dinosaur era. Imagine being able to witness those gigantic creatures up close, running from a T - Rex. Another could be traveling to the future to see how advanced technology has become, like flying cars and buildings that touch the sky. Or perhaps traveling back to a significant historical event and trying to change the outcome for the better.
Well, in some time travel Kakashi fanfictions, there's a plot where Kakashi goes back in time right after Obito's 'death'. He tries to save Obito in a different way, not just by taking his eye. He could change the whole course of events that led to the formation of Akatsuki and the many tragedies that followed. This would completely rewrite the story of Naruto in many ways.
A cool plot idea is Padme time traveling to different eras of the Jedi Order. She could witness the height of their power and also their downfall. Maybe she would meet ancient Jedi Masters and learn from them, then bring that knowledge back to her own time to try and strengthen the Jedi. Or she could travel to a time when the Sith were more dominant and have to figure out a way to survive and return to her time with crucial information.
In some pokemon time travel fanfiction, the main character travels to the future. There, they might find that Pokemon have evolved in unexpected ways, like having new types or abilities. They then have to adapt and figure out how to battle with these new forms of Pokemon.
Another plot might be Kratos traveling to a time in the future of his own world. He could see the consequences of his actions and how the world has changed. Maybe he discovers that his actions have led to a new threat emerging, and he has to figure out how to fix it. This would add a new layer of responsibility to his character.
One interesting plot could be Octavia traveling back in time to prevent the initial war on Earth. She might use her knowledge of future events to broker peace between the different factions. Another could be her traveling to the time before the Ark was built to see if there were other ways to solve the overpopulation problem without having to send people to the ground in such a dangerous way. And she could also time travel to a future version of the world to see what kind of society has been built after all the chaos on Earth in her original time.