A common plotline is Ichigo traveling back to his childhood. He might use his King of Hollows powers to deal with the Hollow that killed his mother in a different way, perhaps saving her. This would completely change his family dynamic and his relationship with his father. Also, he could travel to the time of the Quincy massacre. As the King of Hollows, he might try to stop it, which would have a massive impact on the relationship between Soul Reapers and Quincies in the present.
Some fanfictions might have Ichigo time - travel to a point where the Soul King was first created. As the King of Hollows, he could have a role in that creation or at least witness it. This could give him insights into the true nature of the Soul Society and his place in it. He could then use this knowledge to change things in his own time, like trying to create a more harmonious relationship between Hollows, Quincies, and Soul Reapers.
One popular plotline could be Ichigo traveling back in time to prevent the initial Hollow invasion in Karakura Town. This would change the entire course of events and how characters like Orihime, Chad, and Uryu first got involved in the spiritual battles. Another might be Ichigo going back to the time when the first Soul Reapers were created to understand more about his own powers as a King of Hollows. He could end up influencing the development of Soul Reaper society in ways that have a huge impact on the present he left behind.
In these fanfictions, Ichigo often starts as a more naive character in the past. But as he experiences events again with his new knowledge as the King of Hollows, he becomes more confident. For example, when facing early enemies, he uses his Hollow powers more effectively. His understanding of the complex relationships between different spiritual beings also deepens. He realizes that not all Hollows are simply evil and that there are more shades of gray in the world of Soul Reapers, Quincies, and Hollows.
In some fanfictions, they might time travel to an era where the Quincy were more powerful. Ichigo, being part Quincy, might have to face some difficult decisions regarding his heritage. Orihime would be there to support him, and together they could end up changing the course of history between the Shinigami and the Quincy. This could lead to a completely different world in the present, with new alliances and enmities.
One popular plot could be Ichigo traveling back in time to prevent a major event in the Soul Society's history that had a negative impact. For example, he might go back to stop Aizen's early machinations. Another plot could involve Ichigo traveling forward in time to see the future of his friends and the world he's protecting. He could then come back to the present and work towards a better future. There could also be a plot where Ichigo time travels to different alternate universes where things are very different for him and his friends, like in one universe he never became a Soul Reaper and has to figure out how to get back to his own reality.
One popular plotline is Kirito traveling back to the beginning of SAO. He could use his knowledge from the first play - through to change things for the better, like saving more players from the start. Maybe he'd find a way to defeat the bosses more easily or prevent certain in - game tragedies.
Well, a common plotline is the 'forbidden love' angle. Since they are from different worlds in a sense (Ichigo being a human - turned - substitute Soul Reaper and Soifon being a high - ranking Soul Society member), their relationship might be seen as taboo. So, fanfictions often explore how they deal with this, like trying to keep their feelings a secret from their respective groups. Another one could be a story where Ichigo gets injured and Soifon takes it upon herself to nurse him back to health. During this time, they have deep conversations and get to know each other on a more personal level, leading to the development of feelings. Also, there could be a plot where they are both on a quest for a powerful artifact. Along the way, they face many challenges together and their relationship blossoms as they rely on each other's strengths.
One common plotline is that Ichigo and Naruto end up in a shared universe where they both attract a harem of female characters from their respective shows. For example, Ichigo might have Orihime, Rukia, and others in his harem, while Naruto could have Hinata, Sakura, etc. Another plotline could be a competition between them to see who can build the more impressive harem.
Well, a common plot could be Hollow Ichigo time traveling to key battles in the Bleach universe. For instance, he could go back to the fight between Ichigo and Aizen. He might interfere in the battle in unexpected ways, either helping Ichigo more than he should or causing chaos. Or he could time travel to when the Hollows first emerged and try to change the nature of Hollows from the very beginning. This could have a huge impact on the entire Bleach world and its power balance.
One popular plotline could be Harry traveling back in time to prevent certain tragedies in Tonks' life. For example, he might try to stop her from being killed in the Battle of Hogwarts. Another could be Tonks traveling to a time in Harry's past to offer her unique magical abilities to help him in his early struggles against Voldemort. And sometimes, they both might travel back to a crucial moment in wizarding history to change the course of events that led to Voldemort's rise in the first place.
One popular plotline is Harry going back in time to change key events in the wizarding world. For example, he might go back to prevent certain deaths like Sirius Black's. Another could be him traveling to the past to gain more knowledge about magic from the great wizards of old, which he then uses to fight Voldemort more effectively in his own time.
One popular plotline is that Naruto and Hinata travel back to the time when the Uchiha clan was still intact. They try to prevent the massacre from happening, which changes the whole future of the ninja world. Another plotline could be them going back to their childhood days. Hinata gets a chance to be more confident around Naruto earlier, and Naruto realizes Hinata's feelings sooner, leading to a different relationship development.