One way to do it is to start by thinking about your own experiences and emotions. Maybe a memorable event or a strong feeling could be the basis for a story idea.
One idea could be a conflict between a character's ambition and their moral code. For example, a young scientist is desperate to make a groundbreaking discovery but the only way to do so involves unethical experimentation on animals. This creates an internal struggle as the character has to decide between achieving their life - long ambition and staying true to their principles.
One idea could be a story where the OC is a new student at Rin's school. They meet during a music club event and Rin discovers the OC has a unique musical talent that she's never seen before. This could lead to them collaborating on a special musical project.
One idea could be that the OC is a time - traveling journalist who meets the 10th Doctor during his adventures. They initially clash because the OC is always trying to get a good story, but eventually they become friends and the OC helps the Doctor solve some mysteries. For example, in a planet where the inhabitants are losing their memories, the OC's sharp observational skills come in handy.
One idea could be that the OC is a new Bakugan brawler who enters a tournament. Shun notices the OC's unique battling style and becomes intrigued. They start to train together and develop a friendship that slowly turns into something more. For example, they might discover a new Bakugan ability while training.
One idea could be that the OC is a new villain who teams up with Pythor. They could have a common goal like taking over Ninjago City. The OC might have unique powers that complement Pythor's skills.
One idea is to create an original character who is related to an existing character in the fandom. In a Supernatural fanfic, your OC could be a long - lost relative of the Winchester brothers. This instantly gives them a connection to the main story. Another approach is to make the original character a rival to the main character. In a Bleach fanfic, your OC could be a new Soul Reaper who challenges Ichigo's position as the protector of Karakura Town. And you can also have the OC be an undercover agent in the world of the fanfic, like in a James Bond fanfic, where your OC is a double agent working for a different organization.
One idea could be to have Naruto's chakra system interact with the magic in Divinity Original Sin 2. For example, Naruto could use his chakra to power up certain spells or abilities in the game world. Maybe he teams up with the main characters of Divinity Original Sin 2 to fight a common enemy that threatens both the Naruto universe and the world of Divinity. They could combine their unique skills like Naruto's shadow clones and the Source magic from the game.
A new character could be a rogue Soul Reaper. This character has his own set of beliefs about how the Soul Society should be run and is constantly causing trouble for the Gotei 13. He might have a unique power that allows him to manipulate souls in ways that others can't. His role would be to be a constant thorn in the side of the established order, forcing the main characters to question the Soul Society's rules.
The OC could be a young inventor who comes to Fish - Man Island to study its unique technology. There, they meet Shirahoshi. The OC creates a device that helps Shirahoshi communicate more easily with the surface - dwellers. In return, Shirahoshi shows the OC the hidden secrets of the Fish - Man Island, and they form a strong friendship that transcends the differences between humans and fish - men.