It could be that there is a dark force at work that is manipulating Ichigo's mind. This force convinces him that Rukia holds the key to saving both the human world and the Soul Society from a great catastrophe. So, he kidnaps her with the intention of using her powers in a way he believes is right, but soon realizes he has been tricked when strange things start to happen around them.
One possible plot could be that Ichigo kidnaps Rukia out of a desperate need to protect her from some unseen and extremely dangerous threat in the Soul Society. He might believe that the Soul Society's own mechanisms for safeguarding her are insufficient.
Well, it could be about their baby having some sort of special power. Maybe the baby inherits both rukia's shinigami powers and ichigo's hybrid powers. It could grow up in a world where it has to learn to control these powers while also facing the challenges of being the child of two very important and powerful beings.
Well, one possibility could be that Grimmjow has some personal vendetta against Ichigo. Maybe he wants to use Ichigo as a bargaining chip with the Soul Society. He could be tired of Ichigo always getting in his way during battles and decides to take him to a secluded location where he can deal with him on his own terms.
One idea could be that Aizen kidnaps them to use Ichigo's power and Rukia's knowledge of Soul Society for his own dark plans. He might want to study Ichigo's unique Hollow - Shinigami powers up close and use Rukia as a bargaining chip with Soul Society.
The Soul Society itself could be the main antagonist. If Ichigo kidnaps Rukia because he thinks they are going to harm her unjustly, then the Soul Society's rules and its enforcers would be the opposing force.
One possible plot could be that Ichigo and Rukia are paired for a school project. They start off not getting along very well because of their different personalities. Ichigo is impulsive and Rukia is more reserved. But as they work together, they gradually learn to understand and respect each other. They might face challenges like finding a suitable topic, dealing with difficult classmates who try to disrupt their work, and finally presenting their project successfully which also strengthens their friendship.
It could be about a moment of unexpected closeness between Ichigo and Rukia in a closet. Maybe they are hiding from some danger or just stumbled into this confined space, and this leads to some deep conversations and a new understanding of their relationship.
One possible plot twist could be that Aizen has a hidden motive other than just causing chaos. Maybe he kidnaps Ichigo to use his unique powers for some sort of experiment. For example, he might want to extract the power of Ichigo's Hollow side to enhance his own strength or create a new type of Arrancar. Another twist could be that Aizen is being controlled by an even greater force, and Ichigo's kidnapping is part of a larger plan to draw out other powerful characters in the Soul Society.
In this fanfiction, they might meet differently in terms of their initial encounter. Instead of Rukia being chased by Hollows and crashing into Ichigo's room, they could meet at a spiritual festival. Ichigo is there because he has always been curious about the spiritual world, and Rukia is there undercover to keep an eye on any potential spiritual threats. They strike up a conversation, initially thinking the other is just an ordinary person interested in the spiritual, but soon realize there is more to each other than meets the eye.
The fanfiction could start with the baby girl already being a few years old. She has an in - built talent for seeing spirits like Ichigo. Rukia tries to teach her about the Soul Society while Ichigo wants her to have a normal life in Karakura Town. This creates a conflict between them, and the baby girl has to navigate between her parents' different desires while also dealing with her own emerging powers.
Well, one possible plot development could be the initial shock and confusion for both Ichigo and Rukia. They might struggle to come to terms with the situation, especially considering their complex relationship and the world they live in. There could be a lot of secrecy at first, as they figure out how to handle it. Then perhaps they start to make plans for the future, which might involve protecting the unborn child from the various dangers in their universe.