Caulifla could be a great choice. She's strong - willed and has a passion for fighting. Her energetic personality could create an interesting dynamic with Gohan, who is more reserved in some ways.
The theme of self - discovery is often present. The female Saiyan may be trying to figure out her place in the universe, and Gohan, with his own experiences, helps her. Or Gohan himself might be on a journey of self - discovery with the female Saiyan as a catalyst, especially in terms of his Saiyan heritage.
One popular Gohan and Female Saiyan fanfiction is 'The Saiyan Bond'. It focuses on the relationship development between Gohan and a female Saiyan who arrives on Earth. The story is filled with action as they train together and face common enemies.
One idea could be that the female Saiyan is from a different part of the universe and she and Gohan meet during a galactic tournament. They start as rivals but gradually develop feelings as they see each other's strength and honor.
One idea could be Gohan in his Super Saiyan 4 form facing a new, powerful enemy from another dimension. He could struggle at first to control the raw power of this form while also trying to protect his friends and family. Maybe this new enemy has the ability to nullify the Saiyan's energy attacks, forcing Gohan to rely more on his martial arts skills that he learned from Piccolo.
In some fanfictions, an OC Saiyan could be a time - traveler who meets Gohan at different points in his life. This OC Saiyan could change the course of Gohan's life in small ways, like helping him to train harder during his childhood or giving him advice during the Cell Games that he didn't receive in the original story. This would create a whole new timeline of events for Gohan and the Dragon Ball universe.
In a fanfiction, Gohan as Super Saiyan 3 could be involved in a time - travel adventure. He travels back to a crucial point in Dragon Ball Z history and has to use his Super Saiyan 3 powers to change certain events. This could lead to a whole new timeline where things are different. For example, he might be able to prevent the destruction of Planet Namek or save some characters who originally died.
To write a good Gohan and OC Saiyan fanfiction, start with a strong concept. For instance, you could make the OC Saiyan a rival to Gohan at first but then they become friends. Focus on their interactions. Show how Gohan's kindness and wisdom influence the OC Saiyan, and how the OC Saiyan's boldness or different perspective affects Gohan. Also, don't forget to add action scenes. Since they are Saiyans, there should be epic battles where they show off their powers.
In fanfiction, Gohan acting like a Saiyan can bring out his latent Saiyan pride. He could start to flaunt his power more, not in an arrogant way but as a way to show that he is a true Saiyan. He may also start to embrace Saiyan traditions that he previously ignored, such as the Saiyan love for a good fight and the sense of honor in combat.
One idea could be Gohan struggling to control the new power of the Super Saiyan God. He might accidentally cause some destruction while training and then have to deal with the guilt and the fear of his power getting out of control.