A young girl who has always been fascinated by the story of the Titanic could be the main character. She somehow finds herself transported back in time to the ship. She might be on a mission to save her favorite characters from the movie, like Jack. And through her journey, she discovers a lot about herself and the power of love and friendship on the doomed ship.
Perhaps an old sailor who once heard tales of the Titanic. He has a deep regret in his life and wishes he could have been there to help. Through some mysterious force, he goes back in time to the Titanic. He uses his sailing experience to try and steer the ship away from danger. He also becomes a mentor to some of the young crew members on the ship, sharing his wisdom and trying to change the course of history.
One possible plot for 'Titanic Back in Time' fanfiction is that a modern - day historian goes back in time to the Titanic. They want to study the ship and its passengers up close. However, they end up getting involved in the events on the ship. They might try to document everything in a different way, perhaps to prove some theories they had in the present. And as they interact with the people on the Titanic, they start to form deep emotional connections and struggle with the knowledge of the impending doom.
It could be an ordinary middle - aged person who is dissatisfied with their current life. Maybe they have a boring job and a failing marriage, and turning back time is their chance to start over.
Cell could be a great antagonist. Even if Gohan goes back in time, Cell might still find a way to exist. Maybe he adapts his plan to absorb the Androids earlier in time. Gohan would then have to face a more powerful and unpredictable Cell, who could potentially change the timeline to his own advantage.
Frieza would be a great antagonist. If Goku goes back in time to a point before Frieza was defeated, Frieza could be an even more menacing force as Goku might not have all the power - ups he had in the original timeline. He'd have to rely on his wits and basic training more.
It could be the thief of time themselves. We could get to know their motives, their daily life of stealing time, and their inner thoughts. It would be a very unique perspective.
It could be a time - traveling version of Zeref. He might be trying to stop Natsu from changing things in the past that could potentially lead to his own downfall in the future.
It could be an ancient and powerful Titan who was thought to be defeated long ago. Since Percy is in the past, this Titan could be in his prime and see Percy as a threat to his plans of ruling over the world again. For example, Kronos might be even more powerful in this past era and Percy has to find a way to stop him without the help of his modern - day friends.
Madara Uchiha could also be a main antagonist. If Naruto goes back to a time when Madara was still alive and plotting, Madara would surely oppose Naruto's attempts to change the future. He has his own grand plans for the world and wouldn't let Naruto interfere easily.
It could be Danzo. In the original story, Danzo was a major factor in the Uchiha massacre. If Itachi goes back in time, Danzo would likely still be plotting against the Uchiha. His thirst for power and his extreme methods would put him at odds with Itachi's new attempts to save the clan.
It could be Zeref. If Natsu goes back in time, Zeref might have different plans or be in a more powerful position at that time. He could be the main obstacle for Natsu to overcome, especially if Natsu is trying to change things related to the dark magic that Zeref wields.