Piccolo might also be a savior. He has a soft spot for Gohan since they trained together. He could use his strategic mind and powerful attacks to outwit Raditz and rescue Gohan. Maybe he teams up with Goku for an even more epic rescue mission.
In a 'Gohan kidnapped by Raditz' fanfiction, the plot could involve Raditz trying to turn Gohan against his family. He could expose Gohan to the cruel ways of the Saiyan race that Goku has somewhat distanced himself from. Raditz might show Gohan visions of the Saiyan's former glory and try to convince him that his human family is holding him back from achieving true power.
Goku would be a mix of angry and worried. His Saiyan blood would make him want to rush into battle with Raditz right away. But he also knows how dangerous Raditz is, so he might first try to gather some allies, like Piccolo. He'd be constantly thinking about Gohan and how to rescue him as quickly as possible.
One plot idea could be that Gohan's friends, like Krillin and Bulma, team up to rescue him. They might use some of Bulma's inventions to track Raditz. Another idea is that Gohan manages to outwit Raditz during the kidnapping. Maybe he uses his hidden power to escape Raditz's clutches briefly and leaves a trail for his father Goku to follow.
It could be that Gohan is kidnapped by a group who wants to study the Saiyan physiology. They might be scientists from a different planet or a secret organization on Earth. They take Gohan in order to run various tests on him, not realizing the danger they're in if Gohan manages to break free. And his friends and family then have to go on a mission to rescue him.
It could be a new villain that has emerged, someone who is jealous of Gohan's power and wants to make it his own. This villain might be from a far - off planet and has heard of the Saiyan legend.
If Raditz survived in fanfiction, he would likely have a complex relationship with Goku. He might start to see the value in Goku's way of life. Goku could try to reform him, and Raditz could slowly become a more positive force in the Dragon Ball Z universe.
Raditz could be redeemed by sacrificing himself for the greater good. For example, if there was a powerful enemy threatening the Earth and he steps in to take a fatal blow meant for someone else, like Gohan or Goku.
A good Raditz story should focus on his Saiyan heritage. For example, showing how his Saiyan instincts play out in different situations, like when he's facing a tough enemy or making a decision about whether to show mercy or not.