It could be a fanfiction that delves into the power dynamics. Since Ichigo has three zanpakuto, perhaps it shows how his power has increased exponentially. It might also be about the training he had to go through to be able to handle three zanpakuto, and how this changes his role in the Bleach universe.
One idea could be that the female Zanpakuto has a unique personality. For example, she could be sassy and constantly tease Ichigo while also being fiercely protective of him. In the fanfiction, she might have her own backstory, like being created from an ancient power that was sealed away for centuries until Ichigo awakened her.
It could completely change the dynamics of his powers and battles. Maybe a new Zanpakuto would give him unique abilities or force him to adapt his fighting style in unexpected ways.
In such fanfiction, Ichigo's character might develop a greater sense of adaptability. Since he has a different zanpakuto, he has to learn new ways to fight and control his power. This could make him more resourceful in battles.
Yes, there is likely some out there. Fanfiction platforms often have a wide variety of creative takes on characters. You can start by checking popular fanfiction sites like FanFiction.net or Archive of Our Own. Just search for 'ichigo had female zanpakuto' in their search bars.
The female Zanpakuto could start off as a bit of a mystery to Ichigo. At first, he might not fully understand her power or her intentions. As the fanfiction progresses, she could gradually reveal more about herself, like her past and her true motives for being with Ichigo.
Ichigo would probably be devastated. His trust in relationships would be severely shaken. He might become more withdrawn and focus even more on his battles and training, using it as an escape from the pain of the betrayal. He could also become more cynical about love and people in general.
One possible reason could be to gain more power. In the Bleach universe, the dead often possess unique powers or knowledge. By raising the dead, Ichigo might be seeking to learn from them or use their power to fight stronger enemies.
Another idea for the fanfiction could be a time - travel plot. What if Ichigo somehow travels back in time and meets a younger Rangiku? He could influence her growth and development, and in turn, she could have an impact on his view of the past. This would create a complex web of cause and effect, and it would be interesting to see how their actions in the past would shape the future of the Bleach world they know.