Rocket might also be the one to initiate time travel. He's a genius when it comes to technology, so he could easily figure out how to build a time - travel device. Plus, he's always looking for ways to get an edge in their adventures.
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.
Doctor Strange could be involved in a Guardians of the Galaxy time travel fanfiction. He has extensive knowledge of the mystic arts which could include time - related magic. He might team up with the Guardians and be the one to actually make the time travel happen, perhaps to prevent some sort of universal catastrophe that involves both his world of magic and the Guardians' part of the cosmos.
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.
Well, it could involve the Guardians traveling to different eras in the Marvel universe. They might meet younger versions of themselves or other characters. Maybe they would have to fix something in the past that was messing up the timeline, like Star - Lord accidentally changing a key event that led to Thanos being even more powerful.
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.
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.
I think Tyrion Lannister would be an interesting character to time travel in a fanfiction. He's very intelligent and cunning. Time travel could be his way to outwit his family and those who have wronged him. Arya Stark could also be a likely character. She has the skills of a faceless man. If she could time travel, she could use her skills to change the past, perhaps by assassinating key enemies before they cause harm. And of course, Melisandre. She has some sort of magic related to seeing the future. It's not a stretch to think she could find a way to time travel in a fanfiction and change the course of events in Westeros.
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.
I think Fundy would be likely to time travel. He has a complex relationship with his family and the SMP. Time travel could be a means for him to understand his father better or change the way his family relationships developed. GeorgeNotFound could also be a candidate. He's often in the background but has a strong connection to Dream. Time travel might be his way of getting more involved in the SMP's events and helping Dream in a different way. Lastly, Niki could be a time traveler. She has been through a lot of changes on the SMP, and time travel could be her opportunity to go back and make the SMP a more inclusive and friendly place from the start, especially for those who have been marginalized.
Ban could be another one. He has his own backstory related to the sins and his immortality. He might time travel to find a way to break free from his immortality curse or to go back and correct a mistake he made when he was under the influence of one of the sins, like greed when he stole something important in the past.
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.
Morgana might also be involved. If she were to travel through time, she could either try to change the past to gain more power in the present or she could be sent to the future to see the consequences of her actions. This might lead to a change of heart for her, or it could make her more determined to achieve her goals depending on what she witnesses.