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.
Another plot could involve time traveling to a future where the Portal technology has been misused to the extreme. The main character then has to travel back in time to steal the blueprints of the Portal gun before it was fully developed, so as to prevent this dystopian future from ever happening. They would face challenges like evading the security of the powerful corporations that control the Portal technology in that future.
One interesting plot could be a character going back in time to prevent a certain card from being created, which then changes the whole Yu - Gi - Oh! tournament scene. For example, if the Blue - Eyes White Dragon card was never created, how would the power balance shift among duelists?
One interesting plot could be Superman traveling back in time to prevent the destruction of Krypton. He could use his powers to stop the cataclysmic events that led to its downfall. Maybe he tries to convince the Kryptonian council of the impending danger, but faces resistance as they don't believe him at first.
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.
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.
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.
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.
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.
One interesting plot could be a modern person traveling back to WW2 and using their knowledge of future technology to help the Allies in a covert way, like improving communication systems. Another could be a time traveler trying to prevent a particular tragic event during the war, such as the bombing of a certain city. And there might be a plot where the time traveler has to adapt to the harsh wartime conditions and forms unexpected friendships with soldiers on the front line.
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.
A cool plot in Naruto Konoha 12 Time Travel Fanfiction might be that they go back to the time of the Nine - Tails attack on Konoha. They could work together to try and prevent the Nine - Tails from causing as much destruction. Maybe Sakura, with her medical skills, could save more lives. Shikamaru could come up with a better strategy to deal with the situation. And Naruto, well, he could try to communicate with the Nine - Tails in a different way since he has the experience of befriending it in the future. This would change the whole future of Konoha and how Naruto is perceived in the village.