Another plot could be that an ancient evil force targets Femichigo because she represents a new and unknown power in the Bleach universe. Ichigo vows to protect her at all costs. Along the way, they discover secrets about their connection that were hidden by the Soul Society, and they have to fight against not only the external threat but also the political intrigue within the Soul Society.
The relationship could develop through shared experiences. For example, if they both get trapped in a strange dimension in the Bleach world. They have to rely on each other to survive. Ichigo could teach Femichigo about his human side, and Femichigo could show Ichigo new ways to use his spiritual power. This mutual exchange would bring them closer together.
One plot idea could be that Ichigo and Eragon meet in a dimension that is a cross - between their two worlds. They have to team up to fight a common enemy that is threatening both of their universes. Their different powers, Ichigo's Soul Reaper abilities and Eragon's dragon - riding and magic, would make for an interesting combination in battles.
A plot could involve Ichigo being trapped in the Hollow world and having to find a way back. Along the way, he meets other Hollows with different personalities and motives, some of whom he befriends while others are enemies. He discovers secrets about the origin of Hollows and how it relates to his own powers.
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 idea could be that their pregnancy is a result of some kind of magical or spiritual event in the Bleach universe. Maybe an ancient spell gone wrong or right, depending on how you look at it. For example, a spell that was meant to protect Soul Society actually ends up causing Rukia to become pregnant. Another plot could involve the two of them having to hide the pregnancy at first because it goes against some Soul Reaper rules or traditions. They then have to deal with the consequences of keeping it a secret while also preparing for the arrival of the baby.
One interesting plot idea could be a story where Shirosaki and Ichigo are forced to team up against a new, powerful enemy that neither can defeat alone. This would require them to put aside their differences and learn to work together, leading to some tense and interesting character interactions.
Maybe in the fanfiction, there's a threat that only Ichigo and Unohana can face together. They have to combine their different fighting styles, with Ichigo's brute strength and Unohana's precise and deadly techniques, to overcome the enemy. For example, they might encounter a powerful Hollow that has the ability to nullify most spiritual attacks, so they need to rely on their physical combat skills and teamwork.
One idea could be that the reader is a Quincy and they initially clash with Ichigo due to the historical differences between Soul Reapers and Quincies. But as they face a common enemy, like a powerful Arrancar, they start to understand each other and form a bond. Another could be that the reader has the ability to see into the future in the Bleach universe and warns Ichigo about upcoming threats, and they work together to prevent disasters.
One idea could be that Ichigo has to train under Isane to learn more about healing techniques in the Soul Society. This would be a great way to explore their different skills and how they can learn from each other. Another could be that they team up to rescue a group of young Soul Reapers who are lost in a dangerous part of the Soul Society.
Well, a plot could revolve around Ichigo losing his powers again but Tier being the one to help him regain them in a very unique way. Maybe she has some ancient knowledge or a special connection to a power source that could potentially restore Ichigo's strength. And as they work on this, they start to form a bond that goes beyond friendship. Also, a fanfiction could explore what would happen if Tier was somehow injured and Ichigo had to protect her while also dealing with his own inner demons and the threat of other enemies in the Bleach universe.