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.
One possible plot could be that Ichigo gets sent back in time accidentally due to some of Kisuke's experiments gone wrong. He then has to figure out how to get back to his own time while also dealing with the past versions of his friends and enemies. Kisuke, of course, feels responsible and tries to help him.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Kisuke x Ichigo time travel' in their search bars, and you'll get a list of relevant stories. Some might be really well - written with interesting plot twists and character developments.
One interesting plot twist could be that when they go back in time, they accidentally change a small event that has huge ramifications in the future. For example, they might prevent a key Hollow attack that was originally supposed to happen. This would then change the relationships between characters. Maybe Ichigo doesn't meet some of his friends in the same way because that Hollow attack was a catalyst for many things in the original timeline. Another plot twist could be that they find out there's a hidden force that has been manipulating time all along and they have to fight against it to set things right. And a third could be that when they go back, they discover that their own powers work differently in the past, which adds a whole new layer of challenges for them to overcome.
Friendship turned into a deeper connection might be another theme. In these fanfictions, they often start with a basic acquaintance or friendship and then develop romantic or more complex feelings as the story progresses. Also, themes of self - discovery for both characters can be common as they navigate their new relationship. For example, Ichigo might discover new aspects of his powers or himself through his relationship with Urahara, and vice versa.
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 mentorship. Kisuke often acts as a mentor to Ichigo, guiding him in his powers and understanding of the Soul Society. Another is friendship, as they grow closer through their shared adventures.
A common theme is the growth of their relationship. It could start as a mentor - student type of relationship where Kisuke guides Ichigo in his powers, and then develop into a deeper bond.
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.