One of the main characters could be Captain Lewis. He's highly intelligent and a strategic thinker. He was on the verge of making a major career move in the modern military when they are sent back in time. He has to balance his ambition with the new challenges of the past. Then there's Private Rodriguez, a Latino soldier who brings a different cultural perspective. He's very family - oriented and his thoughts often drift back to his family in the modern day while he tries to survive and make a difference in the past.
A young and inexperienced soldier named Jake could be a main character. He's fresh out of basic training and is suddenly thrown back in time. His naivete in the face of this new and old world at the same time makes for an interesting journey as he has to quickly adapt. Another could be Sergeant Thompson, a tough and battle - hardened veteran who uses his experience to lead the group of soldiers in this strange time - travel situation.
There could be Lieutenant Johnson as a main character. He's known for his cool - headedness under pressure. In the time - travel scenario, he becomes the voice of reason among the soldiers. And also Corporal Davis, a female soldier. She has to face the added challenge of being a woman in a time when gender roles were much more restricted. Her determination to prove herself in this new and old setting makes her a central figure in the fanfiction.
One possible plot could be that a group of US soldiers are sent back in time during a military experiment gone wrong. They end up in a crucial historical battle, like the Battle of Gettysburg. They use their modern military knowledge and advanced weaponry (to a certain extent if they still have it) to change the outcome of the battle, which then has a ripple effect on the course of history.
A young and inexperienced member of the vanguard could also be a main character. Their naivete in dealing with the past could create interesting situations and conflicts. For example, they might accidentally change a small detail that has big consequences.
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, Kaneki himself would be the central character. But also, his friends like Hide could be involved. Hide was always there for Kaneki, and if Kaneki goes back in time, he might want to protect Hide better from the start. Touka could also be a major part. Maybe Kaneki goes back to a time when he first met Touka and tries to change their relationship for the better, and together they could work towards changing the fate of ghouls.
As I'm not familiar with the 'A Time for Us Novel', it's difficult to determine the main characters. However, considering the title, it could be two individuals who are in a relationship, like a man and a woman who are from different backgrounds and their journey to be together could be the focus. Or it could be a family unit where each member plays an important role in a story about family values and togetherness during a certain time.
The main characters are Joel and Ellie. Joel is a hardened survivor who has lost a lot in the post - apocalyptic world. His role is to protect Ellie as they journey across the dangerous world. Ellie is a young girl who is immune to the Cordyceps infection. She might be the key to finding a cure.
Inuyasha could be one of the main characters. Since he has a strong connection with Kagome in the original story, he might also be involved in the time - traveling adventure. Maybe he follows Kagome back in time or is already in the past waiting for her.
There could also be a mentor - like character. This character could be someone from the past that the protagonist meets again when they turn back time. The mentor could guide the protagonist in making different choices this time around.
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.