Well, Naruto is the top one. As the Jinchuriki of the Kyuubi, he's at the center of it all. But don't forget about Jiraiya. He has a huge influence on Naruto and his past self could be a key factor in any time travel plot. And then there's Hinata. Her relationship with Naruto could be explored in new ways if time travel is involved. For example, she could help Naruto in the past in ways she didn't before, adding a new dimension to their relationship.
Naruto is, without a doubt, a central character in Kyuubi time travel fanfiction. His connection to the Kyuubi makes him the prime candidate for time travel. Sakura could also be involved. She might be the one who discovers the key to time travel through her medical ninjutsu research or something. And then there's the Fourth Hokage. His actions in the past are crucial to Naruto's present, so he could be a significant character in time travel scenarios, either as someone to be saved or as a guide for Naruto in the past.
Naruto is usually a main character as he has a connection with the Kyuubi. Since he already has the Kyuubi sealed within him, it's logical that he would be the one to potentially unlock or develop the Kyuubi doujutsu. Sasuke might also be involved. His rivalry with Naruto could lead to interesting interactions in relation to the doujutsu, either trying to obtain it for himself or learning to counter it.
In 'ff8 time travel fanfiction', Zell could also be a main character involved. His energetic and impulsive nature might lead him to accidentally stumble into a time - travel situation. He could then use his fighting skills in different time periods, creating interesting storylines. Laguna Loire might be involved too. Since he has his own mysterious past and connection to Squall, time travel could be used to further explore those relationships and his role in the overall ff8 lore.
Obviously, Revan himself is the central character. But often, characters like Bastila Shan are also involved. Since she was closely associated with Revan in the original story, she might play a role in his time - traveling adventures. Maybe she follows him through time or is the reason he decides to time travel in some fanfictions.
Obviously, Teddy Lupin is the central character. But often, his family members from the past like his father, Remus Lupin, and his mother, Tonks, are also involved. Their relationships and how Teddy interacts with them in the past are key elements of the story.
Often, Percy Jackson might be involved. Since he is a central character in the same universe as Nico, their paths could cross during Nico's time - traveling adventures. Percy could either be a helper or someone who needs Nico's help due to the changes in time.
Well, Ben Tennyson is obviously the main character. He's the one with the Omnitrix and is usually the one doing the time traveling.
Sasuke is also a popular choice. Given his complex character and his quest for power, in a time - travel scenario, he could go back in time to gain more power or change the events that led to the downfall of his clan. Another character is Kakashi. His past and his relationship with Obito could be explored in a time - travel fanfiction. He could try to prevent Obito's fall to the dark side if he had the power to time - travel.
King might also be a character in time travel fanfiction. His search for his true identity could be further explored through time travel. He could go back to the time when his species was more prevalent in the Boiling Isles to learn about his origins, which would be an exciting adventure in the context of time travel fanfiction.
Sasuke could also be a main character in such fanfictions. His quest for power and knowledge could lead him to dabble in time magic. He may want to use it to change his clan's past or gain an upper hand in his rivalry with Naruto.
Obviously, Naruto is the main character as he is the one releasing the Kyuubi. Sasuke could also be involved, perhaps as an ally or an antagonist depending on the plot of the fanfiction. Sakura might be there too, trying to support Naruto in controlling the Kyuubi's power or dealing with the aftermath.