Maybe in a modern AU (alternate universe), Ichigo is a regular high school student and Lisa is a new transfer student. They meet in the library where Ichigo is studying for an exam and Lisa is looking for some rare books. Their initial encounter leads to a series of events that bring them closer together, like working on a school project.
In a post - war Soul Society setting, Ichigo visits the Soul Society often. Lisa, who has always been intrigued by him, starts to show him some of the hidden corners of the Soul Society that not many outsiders know about. As they explore together, their bond strengthens and they realize their feelings for each other.
One idea could be that Ichigo has to go on a special mission with a group of characters and during the journey, they all discover new powers within themselves. For example, Ichigo could learn a new form of his zanpakuto from an unexpected ally.
Maybe in the fanfiction, Ichigo and Naruto meet during a tournament held in a world that combines the elements of their respective universes. They start as rivals but soon realize they have more in common and end up fighting together against those who want to disrupt the tournament for their own evil purposes. This could involve them combining their unique powers like Ichigo's Getsuga Tensho and Naruto's Rasengan in creative ways.
One idea could be a story where Ichigo and Shirosaki are forced to team up against a new, powerful enemy that threatens both their existences. They have to learn to trust each other despite their differences.
One idea could be that Ichigo is somehow transported to the High School DxD world and has to deal with the Devils and Angels there. He might use his Soul Reaper powers in unique ways to solve the problems in this new world.
One idea could be that they face a new and powerful enemy together that is a threat to their entire world. They have to combine their unique abilities in ways they've never done before to defeat it.
One idea could be that Ichigo and Rukia find themselves in an alternate universe where their roles are reversed. Rukia could be the one with the Shinigami powers from the start in Karakura Town, and Ichigo is a normal human who gradually discovers his own latent spiritual abilities to help her. Another could be that they are sent on a special mission together to a realm neither has been to before, like a dimension full of lost souls. There, they have to rely on each other more than ever to find their way back and complete their task.
One idea could be that Ichigo, through some sort of interdimensional rift, ends up in the Avatar world of Legend of Korra. He could initially be seen as a threat due to his otherworldly powers. But as the story progresses, he teams up with Korra to fight a common enemy that is trying to take over both their worlds. His Soul Reaper powers could be used in unique ways, like using his Getsuga Tenshou to blast away dark spirits that are plaguing the Avatar world.
One idea could be that the OC has a special power that only activates when in close proximity to Ichigo. For example, the OC can sense Hollows from a great distance, which helps Ichigo in his battles against them.
One plot idea could be that Ichigo and Eragon meet in a dimension that is a cross - between their two worlds. They have to team up to fight a common enemy that is threatening both of their universes. Their different powers, Ichigo's Soul Reaper abilities and Eragon's dragon - riding and magic, would make for an interesting combination in battles.
One idea could be that Ichigo is somehow transported to the wizarding world. He might use his Soul Reaper powers in unique ways against dark wizards. For example, his Zangetsu could be seen as a sort of magical weapon that even wizards are in awe of.