In the fanfiction, Ichigo turning into a female Arrancar could be a result of a curse. As a female Arrancar, she may find that her appearance is deceiving to her enemies. They underestimate her at first, but she quickly shows her strength. She might have to navigate the complex Arrancar hierarchy, which is different from what she's used to in the Soul Society. Her relationships with her old friends like Rukia and Uryu could also be strained as they don't know how to deal with this new version of Ichigo at first.
Perhaps the story could start with Ichigo being experimented on by a rogue group of scientists or powerful Hollows. They accidentally turn him into a female Arrancar. Ichigo then has to adapt to her new physical form. She might discover new abilities that are unique to female Arrancars, like a different type of Cero or Sonido. Along the way, she could make new allies among the Arrancars who see her as an opportunity for change, and she tries to use her new position to bring peace between the warring factions.
Maybe a corrupt Shinigami officer who has always been against Ichigo. When he hears about Ichigo's transformation into a female Arrancar, he uses it as an excuse to label her as a dangerous outcast and tries to hunt her down. This officer could be power - hungry and see Ichigo as a threat to his own rise in the Soul Society.
I'm not entirely sure as it's a fanfiction, but generally in an Ichigo and Arrancar harem scenario, it might involve Ichigo having some sort of relationship with multiple Arrancar characters. However, without reading the specific fanfiction, it's hard to say exactly what the plot is.
One possible plot could be that Ichigo is hit by a strange Hollow attack that not only changes his physical form to a girl but also gives him new powers. He then has to adapt to this new body while still facing the usual threats in the Bleach universe. His friends might initially be shocked but quickly accept him and help him figure out how to reverse the transformation while still using his new - girl - form - related powers to fight enemies.
One possible plot could be that Ichigo, as a female hollow, struggles to control her new form and its instincts. She might face discrimination from both Soul Reapers and other Hollows. For example, some Soul Reapers might see her as an even greater threat now, while Hollows could try to either recruit or eliminate her.
Love against all odds is also a big theme. Their relationship is often seen as taboo or impossible in the Bleach universe. But they still fall in love. Fanfic writers explore how they deal with the external pressures, like the opposition from their respective groups, and how they still hold on to their feelings for each other. It's a theme that can tug at the heartstrings of readers.
One great arrancar ichigo fanfiction is 'The Hybrid's Path'. It explores an alternate universe where Ichigo has more complex interactions with the arrancars. Another is 'Ichigo's Arrancar Ally' which focuses on an unlikely alliance between Ichigo and an arrancar against a common enemy. And 'Arrancar - Infused Ichigo' shows Ichigo with some unique arrancar - like powers.
In arrancar ichigo fanfiction, the exploration of identity is a key element. Ichigo has his own identity as a Soul Reaper, but when he gets involved with the arrancars, he may question who he really is. Is he still just a Soul Reaper, or is he becoming something more? Also, the power - up scenarios are common. Ichigo might gain new arrancar - based powers that are both a boon and a bane. For instance, he could have trouble controlling them at first. And the moral and ethical dilemmas are important. If he befriends an arrancar, how does he deal with the fact that arrancars are often seen as enemies by the Soul Society? These elements add depth and complexity to the fanfiction.
A big challenge would be trust. Given their backgrounds as former enemies, it would be hard for them to trust each other at first. There could also be cultural differences between them. Ichigo comes from a more human - influenced world in the Soul Society, while Halibel is from the harsh and different world of Hueco Mundo. These cultural differences could lead to misunderstandings and arguments as they try to cooperate. Additionally, there could be external forces trying to keep them apart, like other Arrancars or Soul Reapers who don't want to see them together.
One idea could be that Arrancar Ichigo has to protect Rukia from a new, powerful enemy in Hueco Mundo. Since he has Arrancar powers, he has a different fighting style and perspective, which could lead to some really intense battle scenes and a deeper understanding between them.
A possible plot could involve Ichigo being in a situation where he has to choose between his arrancar nature and his relationships with the harem. His arrancar side might be drawn to power and chaos, but his love for the harem makes him want to stay on a more moral path. This internal struggle could drive the story forward and also affect his relationships within the harem.