The fanfiction could start with Hollow Ichigo being on a rampage. Yoruichi decides to step in to stop him. But instead of a fight, they find themselves in a situation where they are forced to cooperate. As they interact more, a strange bond forms between them. Hollow Ichigo starts to develop a sense of respect for Yoruichi's strength and intelligence, and Yoruichi begins to understand the pain and chaos that exists within Hollow Ichigo.
They could be rivals at first. Hollow Ichigo is a powerful and chaotic force, and Yoruichi is a strong and independent character. Their initial meetings might be full of tension as they test each other's strength.
One possible plot could be that they discover the pregnancy unexpectedly. Maybe they were in the middle of some big battles or adventures, and then they have to suddenly deal with this new situation. Yoruichi, with her experience, might start preparing for the arrival while Ichigo is still in shock but slowly comes to terms with it and becomes very protective.
One possible plot could be them adjusting to married life while still dealing with the spiritual world threats. Maybe Ichigo is trying to balance his duties as a protector and a husband, and Yoruichi is using her experience to guide him in both aspects.
One possible plot could be that Ichigo and Yoruichi are thrown into an alternate dimension where they have to team up to find a way back home. Along the way, they discover new powers within each other and face powerful enemies together.
Well, one possibility could be that Hollow Ichigo's dangerous and alluring nature attracts various characters into a harem. For example, some might be drawn to his power, while others are intrigued by his complex personality. His wild side might make for some intense and passionate interactions within the harem.
One plot twist could be that they face opposition from the Soul Society. Since Ichigo is a substitute Soul Reaper and Yoruichi has a complex history there, the elders might try to stop the marriage for political or traditional reasons.
A plot twist could be that an old enemy of Yoruichi resurfaces, seeing their marriage as an opportunity to get revenge. This enemy could be someone from her past in the cat clan or a Shinigami she had crossed paths with before. Now, Yoruichi and Ichigo not only have to plan their wedding but also defend themselves and their loved ones from this new threat.
One possible plot twist could be that Yoruichi's love for Ichigo is forbidden in the Soul Society. Maybe there are rules against such relationships between different 'classes' or species. For example, Yoruichi being an ex - captain and Ichigo being a human - turned - Soul Reaper - substitute. This could lead to a lot of drama as they try to hide their feelings from others.
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.
Another scenario could be that during their time travel, they get separated. Ichigo ends up in a time where he has to pretend to be a normal human without any spiritual powers, while Yoruichi has to find a way to get back to him and the right time period.