Maybe the OC has a special power or ability. They sense something different about Yuki right away. This leads to a series of events where they are drawn closer together. For example, the OC's power could be related to seeing auras, and Yuki's aura is unique due to his zodiac curse. The OC then tries to find a way to break the curse, which brings them on an adventure full of self - discovery for both of them.
One plot idea could be that the OC is a new student at the school Yuki attends. They accidentally bump into each other in the hallway, and Yuki's polite apology makes the OC intrigued. As the story progresses, the OC discovers Yuki's secret about being a part of the Sohma family and helps him come to terms with his insecurities regarding his place in the family and among his peers.
One idea could be creating an OC who is a long - lost relative of one of the main characters. For example, an OC who is a cousin of Tohru. This OC could bring new family secrets and dynamics to the story.
One idea could be exploring the backstories of the minor characters in more depth. For example, what was Hanajima's life like before she met Tohru? Another could be a 'what if' scenario, like what if Kyo had been accepted by his family from the start. Maybe a time - travel fanfiction where Tohru goes back in time to change some of the sad events in the Sohma family's past.
Perhaps in this fanfiction, the Pokemon gyms are run like the Sohma family houses in Fruits Basket. Each gym leader has a secret related to a Pokemon that is similar to the curse in Fruits Basket. The main character, a young trainer, stumbles upon these secrets while trying to earn gym badges.
In Kyo x Yuki fanfiction, 'character growth through each other' is a popular trope. They can learn from each other's strengths and weaknesses. For example, Yuki can learn from Kyo's more straightforward and passionate nature, while Kyo can learn from Yuki's calm and collected demeanor. 'Mutual understanding' is also a big one. As they start to understand each other better, they form a deeper bond. Additionally, the 'sacrifice for each other' trope can be seen. They might be willing to put themselves in harm's way to protect the other, which shows the depth of their relationship in these fan - created stories.
In many such fanfictions, Yuki's friends would likely launch a desperate search to find him. Kyo might put aside his differences with Yuki in order to rescue him. They would follow any leads, no matter how small. For example, if there was a witness who saw the kidnapper's vehicle, they would track it down. And Tohru would be a source of emotional support for everyone involved, constantly worried about Yuki's safety and trying to keep everyone's spirits up during the search.
The relationship could develop uniquely if the OC is an artist. The OC could see Yuki as a muse and start drawing him. Yuki, who is not used to being the center of someone's creative attention, becomes curious about the OC. This mutual interest in art and the OC's view of Yuki as more than just a student or a zodiac member can lead to a deep and different kind of relationship.
One idea could be a male OC who is a transfer student with a unique playing style that challenges the established teams in the 'Kuroko no Basket' universe. For example, he might have a special ability to read the opponents' moves in an extraordinary way, like being able to predict not just the next pass but also the emotions behind each player's actions, which adds a new layer of psychological warfare to the games.
An idea is for the reader to be a journalist investigating strange happenings in the town where the Sohmas live. They suspect something is up with the family and start to dig deeper. As they get closer to the truth, they form relationships with the Sohmas, especially those who are more guarded about the family secret. This could lead to a story full of mystery and self - discovery for both the reader and the Sohma characters.
One idea could be to have the characters from 'House' end up in the world of 'Fruits Basket'. Maybe House could try to diagnose the curse that plagues the Sohma family. His rational and often brusque approach would clash with the more emotional and superstitious aspects of the 'Fruits Basket' world.
Another concept could be a battle - centered fanfiction. The martial artists from Ranma 1/2 could be challenged by the Sohma family members in Fruits Basket. The Sohmas might use their animal forms to their advantage in combat, while the Ranma characters use their martial arts skills. It could be a series of battles where they learn to respect each other's abilities and in the end, form an alliance of sorts.