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 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 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.
Well, in such a fanfiction, a key plot point could be Ichigo's discovery of his new transcendent powers in the past. Maybe he has to face old enemies with his enhanced abilities. Another point could be how his presence in the past alters the future he came from. For example, his interactions with past characters might change their fates, which in turn could have a butterfly effect on the overall Bleach universe.
Well, of course, Ichigo himself is the central character. But Rukia also often plays an important role. Since she was the one who initially transferred her powers to Ichigo, in the time - travel scenarios, she might be involved in different ways. Maybe Ichigo goes back in time to save her from some danger she faced in the past.
Perhaps in the 'ichigo travels back in time bleach fanfiction', Ichigo travels back to the time when the Soul Society was in a more fragile state. He could use his knowledge from the future to help strengthen the Soul Society against upcoming threats, like Aizen's plans. He might train the younger versions of the Soul Reapers and form new alliances that would change the future battles.
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.
Aizen could still be the main antagonist. Even if Ichigo travels back in time, Aizen's long - term plans might be already in motion. Ichigo would have to find a way to expose and stop Aizen earlier than in the original timeline.
One plot idea could be that Ichigo goes back in time to the moment before a major battle in the Soul Society arc. He uses his knowledge of the future to prevent certain casualties and change the course of the battle in a more favorable way for his side. Another idea is that he travels back to his own childhood and tries to understand his powers better from an earlier age, which could lead to him becoming even stronger in the present. Or perhaps he goes back to a time when the Hollows first started emerging in large numbers and tries to find the root cause of their sudden increase, which could potentially change the whole history of the Soul Reaper - Hollow conflict.
Ichigo might become more cautious. In his own time, he's often impulsive, but going back in time and seeing the consequences of actions he hasn't even made yet could make him think twice. He might also gain a deeper understanding of the history of the Soul Society and his own place in it. Kisuke, on the other hand, could become more open. He usually has his own secrets, but in the past, he might be forced to be more transparent with Ichigo and others in order to survive and complete their mission. For example, he might have to reveal some of his early experiments or knowledge that he's been keeping to himself.