For Simon, if he travels back in time, he would be a different person than he was in the original timeline at that point. He'd have the wisdom of his future experiences but also the naivete of his younger self. So you'd have to show this internal conflict. For example, he might be tempted to use his future knowledge too much and risk changing things for the worse.
Kamina in a time - travel scenario would be quite interesting. He's so full of bravado, but if he travels to a time where things are different, he might initially be confused. However, his core personality of being a fearless leader would still shine through. You could write him as being a bit more cautious in some situations, as he now knows there are greater threats out there from his time - travel experiences.
One idea could be that Simon travels back in time to the beginning of the spiral war. He could use his knowledge of future events to better prepare the Dai - Gurren Brigade, leading to a quicker victory against the Beastmen. Maybe he'd also find a way to save some of the characters who died in the original timeline.
For Sakura, her character can be developed by showing how she adapts to the different time periods. In the past, she might be more naive compared to her future self. When she travels to the past, she could learn from her own experiences in the future and become more confident in using her skills. For Neji, if he travels to the future, he could be initially shocked by the technological advancements and different social structures. This could make him question his own beliefs and values, leading to growth in his character.
Merlin would likely become more cautious. In a time - travel situation, he'd know that his actions could have huge consequences for the future. For example, if he accidentally reveals too much about his magic in the past, it could change everything. Morgana, on the other hand, might see an opportunity for power. She could try to use her knowledge of the future to gain a stronger position in the past.
Both of them would probably grow closer as they rely on each other in this strange new situation. They'd have to trust each other even more as they navigate through time. Their shared experiences in different time periods would give them a unique bond. They'd also have to adapt to different social norms and technologies in different times, which would force them to develop new skills and ways of thinking, and this would in turn shape their characters in different ways.
Baymax would need to adapt to different time periods. His knowledge base might be limited in some historical times. He could learn new things and update his database, which could change his personality a bit. Maybe he'd start to have a more historical perspective in his responses.
If ASL time - travels, Luffy's relationship with his brothers would be strengthened in new ways. They would have to rely on each other more as they face the unknowns of the past. For example, Ace might see a younger Luffy and be even more protective.
GoGo might become more patient. In a time - travel situation, she'd have to deal with the differences in technology and culture of the past. This would force her to slow down and adapt, which could lead to a change in her typically impatient personality. She might start to appreciate the process more rather than just focusing on the end result.
For Zephyr, you could write her as being very curious about the different time periods. She might be the one who is always eager to explore and ask questions. For example, when they travel to an ancient Viking time, she would be quick to learn about their customs and try to participate.
For Logan in a time - travel fanfiction, keep his core traits intact. His gruffness, loyalty, and his inner struggle with his past should still be there. But add new layers based on the time - travel experience. For example, if he travels to the past, he might be more cautious as he knows the future consequences.
Their relationship could become more of a teacher - student dynamic. Goku could teach Android 18 about the ways of the Saiyans and their fighting techniques from different eras during the time travel. Android 18, in turn, could show Goku some of her android - specific skills and how to use them more effectively. This exchange of knowledge could bring them closer and make them a more formidable team.