Ciel is a likely candidate. Given his intelligence and determination to avenge his family and protect his future, he might see time travel as a means to change certain events in the past. He could use it to find out more about the people who wronged his family and stop them before they act.
Sebastian could also be a character to initiate time travel. As a demon, he has certain powers and knowledge that might enable him to manipulate time. Maybe he discovers a way to travel through time in his pursuit of souls or to fulfill his contract with Ciel in a more efficient way.
Batman himself is a likely candidate. Given his intelligence and determination, he might stumble upon a time - traveling device or technology while investigating some strange phenomenon. He would be driven by his need to protect Gotham, perhaps by preventing a future catastrophe that he has foreseen.
Star - Lord is a likely candidate. He's always been a bit of a risk - taker and his curiosity might lead him to experiment with time travel technology.
One interesting concept could be how the characters' relationships change due to time travel. For example, Sebastian might have to adapt to a different era's social norms while still serving Ciel, which could add new layers to their dynamic. Another concept could be the alteration of the plot. If Ciel travels back in time, he could potentially prevent certain tragedies from occurring in his family's past, completely changing the story's trajectory.
Koro - sensei could be a likely candidate. Since he has advanced abilities, he might be able to figure out time travel in some way. He could use it to try to change his own past or to better prepare the students for their future.
Lelouch would be a likely candidate. He has a strong sense of justice and a desire to change the world. Given his intelligence and strategic mind, if he were to discover the means of time travel, he would likely use it to correct the many wrongs in the Code Geass world, such as the oppression of Britannia. He could go back in time to prevent his mother's death and change the course of his own life and the fate of the world.
Lelouch is a very likely candidate. Given his intelligence and his desire to change the world, time travel could be a means for him to correct past mistakes or gain more power to overthrow Britannia. Another one could be Suzaku. He has a strong sense of justice and might use time travel to prevent some of the tragedies that occurred.
Katsuki Bakugo, with his powerful quirk and complex personality, could be a character in time travel fanfics. He might go back in time to prove himself even more or to change certain interactions he had with other characters in the past.
I really like 'Through Time in the Black Butler Realm'. In this fanfic, the time travel aspect is used to explore different relationships in the series. It's not just about the main characters but also delves into the backstories of some of the side characters. The way the author weaves the time travel concept into the existing 'Black Butler' lore is quite engaging.
One way could be through some sort of wormhole encounter. If a flight accidentally flew into a naturally occurring wormhole during its journey, it might be transported to a different time. Wormholes are theoretical shortcuts in space - time, so a flight entering one could end up in the past or future.
Well, some RWBY timetravel fanfictions use advanced technology for time travel. There could be a mad scientist - type character who creates a time - travel device. This device might be a large, elaborate machine that the characters step into and are then transported through time. Also, in certain fanfictions, there are time - related Grimm. These Grimm have the ability to distort time around them, and if the characters get too close or are affected by their powers, they end up traveling through time.