For Yamcha, you can focus on his growth from being a bit of a comical character to a more serious and respected fighter in the context of his relationship with Frieza. Maybe he learns from Frieza's cold and calculating nature how to better strategize in battles. For Frieza, his development could be about him learning to care for someone other than himself. He could start to see Yamcha as an equal, which is a huge shift from his usual superiority complex.
You can also make him have a unique role in the story. For instance, he could be the one to discover a new threat to the Earth before anyone else. He then has to convince the other Z - Fighters to take this threat seriously. This would put him in a leadership position and make him stand out more.
No. Abuse is never an appropriate topic, especially when it involves beloved characters from a popular series like Dragon Ball. It goes against the positive values that the series usually promotes.
Power dynamics is a common theme. Yujiro is often depicted as extremely powerful, and the relationship with the OC can be shaped by this. For example, the OC may be in awe of his strength or try to prove themselves equal to him.
When writing a Harry Potter and Yujiro Hanma fanfiction, it's important to establish the rules of the combined world early on. Will Yujiro be able to use magic? If not, how will he defend himself against magical attacks? You can also play with the idea of cultural differences. Yujiro comes from a very different kind of world compared to the wizarding world. This can lead to some interesting misunderstandings and comical situations. For instance, Yujiro might think the Sorting Hat is some kind of strange creature trying to attack him and try to fight it off. As you write, keep the pacing right, not too slow or too fast, and make sure the story has a satisfying ending, whether it's a happy ending or a more bittersweet one.
One interesting Yujiro Hanma fanfiction could be about his early days of training. It might explore how he developed his extreme strength and fighting skills. Maybe it shows his encounters with various opponents during his youth and how he overcame them to become the fearsome fighter he is known as.
A popular Yamcha fanfiction is 'Yamcha's New Beginnings'. In this one, Yamcha decides to start anew after feeling left behind by his friends. He goes on a solo adventure, meets new allies and enemies, and in the process, he redefines himself and his place in the Dragon Ball universe. It gives a fresh perspective on his character.