Another theme could be the exploration of their different worlds. Ichigo is from the human world and Soul Society, while Halibel is from Hueco Mundo. Fanfictions might delve into how they bridge the gap between these two very different realms, perhaps through mutual understanding and cooperation. This can lead to interesting stories about cultural exchange in a sense, within the 'Bleach' universe.
Their powers could be combined in a way where Ichigo's Zanpakuto spirit, Zangetsu, and Halibel's water - based powers merge. For example, Ichigo could gain the ability to use water - enhanced slashes that have the cutting power of his normal attacks along with the fluidity of Halibel's water attacks.
One common theme is power struggle. Given that both characters are quite powerful in their own rights, fanfictions often explore how they deal with each other in terms of strength. Another theme could be the exploration of the dark side of their personalities, as Hollow Ichigo is, well, hollow, and Halibel has her own complex nature.
One common theme could be the exploration of their unique relationship. Since they are from different 'worlds' in the original context, fanfiction might play with how they interact and understand each other.
Perhaps a plot where Ichigo accidentally ends up in Hueco Mundo near Halibel's territory. She initially sees him as a threat but then discovers that he has no intention of harming her or her fracciones. This could lead to an exploration of their different worlds and cultures within the Bleach universe.
One plot idea could be that Ichigo and Halibel are forced to team up against a new, powerful enemy that threatens both their worlds. They start off distrusting each other but gradually learn to respect and rely on one another's strengths.
One common theme is power. Since Halibel is a powerful character, many fanfics explore her power further, like how she gains more power or how she uses it in different situations.
For Ichigo, he could develop a more nuanced view of the Arrancars. Through his interactions with Halibel, he might realize that not all Arrancars are pure evil. This could make him more open - minded in his future encounters with other Hollows. As for Halibel, she could start to see the value in the Shinigami way, especially Ichigo's sense of justice. It might make her question her own loyalties and purpose.
One common theme is growth. Since Ichigo is depicted as little, his journey of becoming stronger and more aware of his powers is often explored. Another theme could be friendship, like how his relationships with his friends in Karakura Town develop at an earlier stage.
One common theme is the struggle to regain strength. When Ichigo is injured, he has to fight his way back to his former power. Another theme is the support from friends. His comrades in Bleach, like Orihime and Chad, often play important roles in his recovery in fanfics.
One common theme is empowerment. Since Ichigo is often portrayed as a strong male character in the original, when she's female in the fanfic, it's about showing her strength in different ways. For example, she might face different challenges because of her gender but still overcome them.