A plot could be that Haku loses his memory and Chihiro has to help him remember who he is. In the process, she shows him all the things they did together, and their relationship is rekindled in a new way. This would involve her describing their first encounter, the times they spent together, and how he helped her in the spirit world. It would be a story full of emotion and reminiscing.
Perhaps there is a new threat in the spirit world that only Haku and Chihiro can defeat together. As they work side by side, they start to develop romantic feelings for each other. Their unique abilities complement each other, and through their shared experiences in fighting the threat, they grow closer and closer.
One idea is that Chihiro gets trapped in a different part of the spirit world and Haku has to use all his powers to find her. Along the way, he realizes just how much she means to him.
One idea could be a story about Haku and Chihiro reuniting in the spirit world years later. Chihiro has grown up but still remembers Haku vividly. Another could be a fanfiction where they go on an adventure together outside of the bathhouse to explore other parts of the spirit realm that were previously unknown to them.
A fanfic could focus on a journey that Haku and Chihiro take together within the spirit world that they didn't get to do in the movie. They could face new challenges and in the process, their relationship deepens as they rely on each other for support.
One great fanfic could be 'Chihiro and Haku's New Adventure'. It might explore their relationship further after the events of the movie, like them exploring new parts of the spirit world together.
One possible plot could be that Haku and Chihiro meet again in the spirit world years after Chihiro's first visit. As they reunite, they start to explore their feelings for each other more deeply. They go on adventures together, facing new spirits and challenges, and their relationship gradually blossoms into a romantic one filled with tender moments and mutual understanding.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fan - created works, and you're likely to find many Spirited Away fanfics there, including ones about Chihiro and Haku.
In the fanfic, their relationship could develop in a rather magical way. Haku, being a spirit, has a different view of the world compared to Chihiro. But as they interact more, they start to see things from each other's perspective. Maybe they start with simple exchanges like sharing a meal in the spirit world. Then, as time passes, they find themselves drawn to each other. Haku might show Chihiro secret places in the spirit world, and these shared experiences make their bond stronger. Eventually, they develop a love that's both pure and powerful, full of little affectionate moments like stolen glances and gentle touches.
One plot idea could be that the OC is a spirit from a different part of the spirit world who comes to Yubaba's bathhouse. Haku is immediately intrigued by this new spirit and they start to form a bond as they both deal with the chaos in the bathhouse. Another idea is that the OC has a special power that can help Haku break free from Yubaba's control, leading to a journey of self - discovery for both of them.
Their relationship could be developed by having them communicate more deeply. For example, Haku could share more about his past and his true form with Chihiro. This would create a stronger bond between them as Chihiro would understand him better.
One possible plot could be that the kiss is a moment of realization for both Chihiro and Haku. It might be a way for them to break a spell that has been holding Haku in some sort of limbo. After the kiss, Haku could start to remember more of his true self and his past, which could lead to him being able to finally leave the spirit world and find peace. And for Chihiro, it could be a sign of her growing up and understanding her feelings in this strange world she has found herself in.
Well, it would likely be a very different take on the Spirited Away universe. It might explore how their relationship would change with such a big development. Maybe it would show them facing challenges in the spirit world while also dealing with the new life they are about to bring into the world. However, it's important to note that this is a departure from the original story's tone.