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.
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.
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.
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.
One common theme is friendship. The male OC often forms deep friendships with the characters from the original 'Fruits Basket' story. For example, he might befriend Tohru and help her understand the Sohma family better.
One idea could be to focus on a character who feels like an outsider in the Sohma family and their journey to find their own 'solitary freedom'. For example, they might discover a hidden talent or passion that sets them apart from the family's usual drama.
Love and acceptance are common themes. The OC usually has to accept Kyo's curse and he has to accept her into his life. Their relationship grows as they learn to accept each other's flaws.
One interesting storyline could be Inuyasha ending up in the world of Fruits Basket. He might initially be confused by the strange curse and the family dynamics. For example, he could mistake the Sohma's transformation for some kind of demon magic he doesn't understand. His brash nature might clash with some of the more reserved Fruits Basket characters like Yuki, leading to some humorous and tense situations as they try to figure out how to send Inuyasha back to his own world.