One common theme is redemption. Ichigo might go back in time to fix his past mistakes, like not being able to save someone. Another theme is power - up. Since he has the knowledge of the future, he can train differently in the past and gain stronger powers. Also, relationship repair is a theme. He could go back to mend broken relationships with his friends or family.
One popular 'ichigo goes back in time fanfiction' is 'A New Beginning'. In this story, Ichigo goes back to a crucial point in his past and uses his knowledge from the future to change some fateful events. Another one is 'Time - Rewritten'. Here, Ichigo's time - travel causes a ripple effect that alters the relationships with his friends and enemies in unexpected ways.
She could meet her younger self. This would be an interesting encounter as she could offer advice and guidance to her younger version. Maybe she could help her younger self to be less afraid of her powers.
Ichigo's character may develop by becoming more confident in his powers. In the past, he has a chance to redo things and learn from his mistakes. He could become more strategic in battles, using his transcendent abilities more effectively.
One interesting plot could be Ichigo going back in time and preventing certain tragedies that happened in the original story. For example, he might stop the death of a beloved character, which would then change the entire course of events and relationships in the Bleach universe.
One potential ally could be a friendly youkai student who is intrigued by Ichigo's godlike powers. Maybe someone who is an outcast among the youkai and sees Ichigo as a kindred spirit.
There could be some of the more benevolent yokai who see Ichigo as a potential ally. For example, the werewolf Koga might recognize Ichigo's strength and see an opportunity to team up with him against common enemies. Maybe even some of the teachers at the academy who are tired of the chaos and power plays could join forces with Ichigo.
He could change the course of major events. For example, he might prevent the rise of a powerful villain. This would have a huge impact on the entire galaxy. Maybe he could save some characters who died in the original timeline.
One interesting aspect could be that Luke goes back to a time when the Sith were just emerging. He could try to root out the Sith threat before it became as powerful as it did. He might have to be very careful not to disrupt the timeline too much, but his Jedi wisdom could be a great asset in this time - traveling adventure.
In this fanfiction, Sora might change key events. Maybe he could prevent some of the tragedies that occurred in the original story. For example, he could save a character who was lost before.