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.
It could be that a powerful enemy sends Ichigo back in time as a way to get rid of him. Kisuke, being the clever character he is, figures out what happened and follows Ichigo. In the past, they have to find a way to not only defeat this enemy's past self but also find a way to return to their own time. During this time travel adventure, they learn more about each other and themselves.
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 possibility could be that Ichigo is betrayed by a close friend during his time - traveling adventures. Maybe this friend is jealous of Ichigo's powers or has been influenced by an enemy. This betrayal could lead Ichigo to question his relationships and his purpose in the time - traveling journey. He might then have to find a way to overcome this betrayal and still fulfill whatever mission he has in the different time periods.
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.
One possibility could be that during his time travel, Ichigo discovers that his friends or allies in the future have turned against him for some reason. Maybe they believe he has become too powerful and is a threat. So they plot to betray him, which sets up a conflict where Ichigo has to fight against those he once considered friends while also trying to figure out a way to fix the timeline.
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.
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 interesting plot point could be how they interact with their past or future selves. For example, in some fanfictions, Ichigo might meet his future self who has different views on his relationship with Rukia, which causes him to reevaluate things. Another is how they deal with the changes in the timeline. If they accidentally change something in the past, they have to figure out how to fix it without ruining their relationship.
Perhaps the time travel is not what it seems. It could be a trap set by an unknown enemy. When Ichigo and Unohana think they are traveling through time to fix something, they are actually being led into a situation where they will be separated and used against each other. This would create a lot of drama and conflict in the fanfiction.
The plot could involve a fusion of their powers. Naruto's chakra and his jutsu combined with Kisuke's abilities could create a really powerful force. There could also be a plot where Naruto is mistaken for someone in Kisuke's world and has to clear his identity while also trying to figure out how to get back to his own world. Along the way, he forms new friendships and rivalries in this new world.