I'm not sure if there is such a specific fanfiction. Fanfiction can cover a wide variety of scenarios, but this particular one might be a bit rare or not in line with the general tone of the original work. You could try searching on some major fanfiction websites to see if it exists.
While the world of fanfiction is vast and full of creative and sometimes out - there ideas, this particular concept seems rather unorthodox. It's important to note that if such a fanfiction exists, it would likely be a very niche creation. And given the nature of the Bleach characters and their relationships in the source material, it's not something one would expect to find easily in the realm of fan - created stories.
In some fanfictions, it might be because of jealousy. For example, if Rukia spends a lot of time with another character, Ichigo could get pissed. He might have feelings for her that he hasn't fully realized or admitted, and seeing her with someone else stirs up his anger.
One possible plotline is that Ichigo discovers Nejibana in a hidden location. Maybe it's in an ancient temple in the Soul Society that has been long forgotten. He senses its power and decides to take it, not fully aware of what it can do. Then he has to figure out how to use it while being chased by those who don't want him to have such a powerful weapon.
In this fanfiction, Ichigo getting Nejibana could change the entire dynamic of the Bleach universe. Nejibana might have a unique ability that no other weapon has. Ichigo would need to learn how to control it. This could involve some training with characters who are more familiar with such weapons. The story could also explore how his relationships with other characters change. His enemies might be more afraid of him, while his friends might look at him in a new light. There could be a lot of action scenes where Ichigo shows off the power of Nejibana, like when he's facing a horde of Hollows or a powerful Arrancar. And as he gets more used to the weapon, he could even discover new forms or techniques that are unique to Nejibana.
It would be a huge departure from the original Bleach narrative. Ichigo has his own set of relationships and character arcs, and Yachiru is also part of a different set of storylines. Such a fanfiction would likely shock and divide the fan base.
One possible plot could be a post - battle scenario. After all the chaos and fighting they've been through, Ichigo realizes his true feelings for Rukia. They start to secretly date while trying to maintain their duties as Soul Reapers and protectors of the living world. Eventually, their friends find out and support them wholeheartedly. They plan a simple yet beautiful wedding in the Soul Society, with all their friends and comrades in attendance, and they live happily ever after, constantly going between the Soul Society and the living world to visit their loved ones.
The plot could involve Nemu saving Ichigo's life during a fierce battle. This act makes Ichigo see Nemu in a new light. They begin to spend more time together, sharing their past traumas and dreams. As they fall deeper in love, they decide to get married. However, they have to keep it a secret at first due to the political situation between Hueco Mundo and Soul Society. Eventually, their love story becomes an inspiration for others in both worlds.
A plot twist might be that Rangiku discovers a long - lost family secret just before the wedding. For example, she could find out that she has a connection to a powerful and mysterious clan that has some sort of vendetta against Ichigo's family. This would put a strain on their relationship as they try to figure out how to deal with this new information.
Well, a really interesting plot twist could be that Yoruichi is actually a reincarnation of an ancient soul that was once bound to Ichigo's family in a very different way. And their marriage is a way to restore some sort of balance in the world of Bleach. Also, what if Ichigo suddenly starts to lose his powers during the wedding preparations? It could be due to some sort of curse or a side - effect of their impending union. Then they have to figure out how to get his powers back without cancelling the wedding.