Writing a 'badass gohan fanfic' can be really exciting. You need to have a clear understanding of Gohan's backstory, his family, and his relationship with other characters. This will help you create a more immersive story. For the plot, you could consider having Gohan go on a solo adventure to a new planet where he discovers a hidden power within himself. When writing about his badass moments, use powerful language. For example, 'Gohan's ki flared up, a brilliant golden light that sent shockwaves through the battlefield. His presence was like that of a god, and the enemies cowered before him.' Also, add some emotional depth to the story. Maybe Gohan is fighting not just for himself but also for the safety of his loved ones.
First, know Gohan's character well. Think about his strengths like his Saiyan power and his intelligence. Then, create a cool and challenging situation for him, like a new super - villain he has to face. For example, you could make the villain have a unique power that forces Gohan to think outside the box. Also, don't be afraid to give Gohan some new moves or abilities. Just make sure they fit his character.