Another option could be 'Saving 80,000 Gold in Another World for My Retirement'. Here, the MC finds themselves sent back in time to a different world with a chance to start anew.
Well, Percy is obviously one of the main characters since it's about him getting sent back in time. There might also be his friends like Annabeth. In the past, he could meet characters who are important in the history of his world, like the ancient gods or heroes. They would play significant roles in his journey back in time as he might need their help or they could be obstacles in his way.
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.
Percy Jackson would be an obvious choice. His skills as a demigod and his experience with different quests would make him a great character to be sent back in time. He could use his water powers in unique ways in different historical settings.
Obviously, Percy is one of the main characters. But there could be others depending on the specific plot of the fanfiction. Maybe there are some characters from Percy's past that play important roles.
One great 'Percy sent back in time' fanfic is 'Back to the Beginning'. It shows Percy having to deal with all the early threats in a new way. Another is 'Time - Twisted Percy' which has an interesting take on how his knowledge of the future changes things. There's also 'Percy's Time - Travel Adventure' where he forms unexpected alliances in the past.
There are many such light novels. One example could be 'The Time-Traveler's Adventure' where the MC goes back to fix past mistakes and change the future.
Obviously, Naruto himself is the main character as he is the one sent back in time. Sasuke is also often involved as Naruto's relationship with him is a central part of the Naruto story. Kakashi could be involved too, as Naruto might seek his help or try to influence his training methods. Also, the various Hokages might be important characters as Naruto could interact with them in the past to change the course of events.
If Anakin is sent back in time, he might become more cautious. Knowing what his future holds, he would be careful not to repeat the same mistakes. His character could develop into a more patient and wiser version of himself. He might also become more empathetic as he has a second chance to understand the consequences of his actions on others.
He could become a more benevolent figure. Knowing the destruction the Sith can cause, he may focus more on spreading the light side of the Force. He might use his past experiences to teach younger Jedi about the dangers of the dark side in a more effective way than was done in the original timeline. This could lead to a stronger and more united Jedi Order in the long run.
His character would likely become more mature. Seeing the past and knowing the future would make him more responsible. He might not be as reckless as he was before.