Well, one possible fanfic could be about Android 18 and Raditz teaming up against a common enemy. Maybe they initially start off as reluctant allies, but as they fight side by side, they begin to respect each other's strength.
Perhaps in the fanfic, Android 18 uses her combat skills to train Raditz. Since she has a lot of fighting experience, she could help Raditz improve his techniques. This could lead to an interesting dynamic where Raditz becomes more powerful under her guidance.
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.
Common themes in 'Dragon Ball Z Raditz x Bulma fanfic' often include the clash of their different worlds. Raditz comes from a warrior - based Saiyan culture, and Bulma from a more technologically advanced Earth culture. So there could be a theme of how they learn from each other. Also, themes of love against all odds can be present, as their relationship would be quite unorthodox in the context of the original Dragon Ball Z story.
If Raditz turned good, it would have added a whole new dynamic to the Dragon Ball Z universe. He could have become a valuable ally, sharing his knowledge and skills to help the heroes.
Perhaps the fanfic could be a time - travel story. Android 21 accidentally sends Goku back in time to a point where she was first created. Goku then tries to prevent her from becoming the villain she was meant to be. This would involve Goku having to deal with the scientists who created her and convincing them to take a different path in their research, all while trying to find his way back to his own time.
It could also be a fanfic centered around their friendship. Android 18 might initially be cold towards Bulma, but as time goes on, they start to bond. Bulma could introduce Android 18 to the joys of normal life, like shopping or going to the beach. And Android 18 could protect Bulma from threats that her scientific work sometimes attracts. In the end, they become inseparable friends, changing the way the other characters view Android 18.
Goku is very important. He is the main protagonist and the one Raditz is after. Piccolo is also crucial as he teams up with Goku against Raditz, which is quite unexpected considering their past. And of course, Raditz himself is a key character as he is the main antagonist in this part of the story.
There could be a fanfiction where Android 17 and 18 are sent to an alternate universe. In this universe, they have to team up with their evil counterparts to defeat a greater threat that endangers all universes. It would be a story full of action and character development as they deal with their doppelgangers.
In a fanfic, it could be that Android 21 has a hidden power that only Goku can unlock. As Goku helps her discover this power, they form a deep bond. This power could be related to the Dragon Balls in some way, like having the ability to control the Dragon Balls' wishes or even create new Dragon Balls.
The main plot points start with Raditz's sudden appearance on Earth. His presence shakes Goku's world as he learns about his true heritage. Their confrontation is a big part of the plot. During the battle, the power struggle between Raditz's Saiyan strength and Goku and Piccolo's combined might is intense. Goku's decision to sacrifice himself by holding Raditz still for Piccolo's special beam cannon not only defeats Raditz but also sends a message about Goku's selfless nature. This event also leads to the introduction of the concept of the Saiyan's ability to come back to life, which is important for the overall Dragon Ball Z storyline.