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.
Another plot could be that the OC is a healer. In a world full of battles against Hollows and other powerful enemies, Ichigo often gets injured. The OC's role is to heal him and gradually, they fall in love. Their relationship is tested when the OC has to use her healing powers to save Ichigo from a near - fatal injury, sacrificing a part of her own spiritual essence in the process.
Well, a plot could involve Ichigo being turned into a vampire against his will. He then has to deal with the internal conflict of his human morals versus his new vampiric instincts. He might find that some of his friends are scared of him, while others are determined to help him find a way to reverse the transformation. And there could be a love interest who accepts him despite his new nature, adding a romantic sub - plot to the story.
One idea could be Ichigo getting trapped in a virtual reality game world where he has to fight his way out using his Soul Reaper powers. Another could be a crossover where Ichigo teams up with other game characters to defeat a powerful in - game boss.
One idea could be that Ichigo has to hide his vampire nature from his friends at first. He struggles with the new instincts and powers, like enhanced speed and strength. Maybe he has to resist the urge to drink blood from his loved ones.
One idea could be Ichigo crossing over into a different anime universe like the Naruto world. He could meet Naruto and they'd team up against a common enemy. Maybe a powerful new threat that combines the powers of Hollows and the Tailed Beasts.
Well, a cool crossover fanfiction could be with the DC universe. Imagine Ichigo meeting Superman. They would surely have different views on justice. Superman is all about truth, justice, and the American way, while Ichigo is more about protecting his friends and the spirit world. Their meeting could lead to some intense debates and also team - ups against powerful villains.
One interesting story could be where Ichigo and Yachiru go on an adventure in the Soul Society together. They might encounter some strange spirits and have to use their combined skills to defeat them. Ichigo's determination and Yachiru's energetic nature would make for a great dynamic during the adventure.
One interesting plot could be a story where the reader has some sort of special power that Ichigo discovers. He then tries to help the reader control it while they also face various Hollow threats together. This creates a bond between them that goes beyond the normal hero - sidekick relationship.
When it comes to the more complex plot points, the interaction between Ichigo's Hollow side and the Dark Magic in the Harry Potter universe is very engaging. For example, if his Hollow side was somehow influenced by the Dark Arts or if it could be used to fight against it in a unique way. This could lead to a whole new set of challenges and character development for Ichigo within the wizarding world.
One idea could be a crossover where Ichigo and Naruto are transported to a strange new world together. They have to combine their skills, like Ichigo's Zanpakuto abilities and Naruto's ninjutsu, to find a way back home.
Another fanfiction could be a story where Sakura, for some reason, is transported to the world of Bleach. She enrolls in the Soul Reaper academy, where she meets Ichigo. They start off as rivals, constantly competing in combat training and academics. But as time passes, they realize they have a lot in common and end up becoming good friends and allies.