A great plot idea would be Android 21 crossing into a world where technology is feared. She'd have to prove that not all advanced technology is bad. Also, she could cross into a world of pirates, using her skills to navigate the high seas and fight off other pirates.
Well, a plot could be Android 21 somehow getting her hands on the Namekian Dragon Balls. Since she has that scientific mind, she might try to figure out how to modify the wishes. In the process, she could draw the attention of Frieza who wants the Dragon Balls for his own evil purposes. Then the Z - fighters would have to step in to protect Android 21 and the Dragon Balls. Another plot idea is that Android 21 is from an alternate universe where she was created by the good guys. When she crosses over to the main Dragon Ball Z universe, she has to adjust to the different power levels and politics among the characters. And she could end up being a key player in the fight against Buu, perhaps using her unique abilities to weaken Buu in some way.
One idea could be Android 21's struggle to fit into a world that fears her. She has to prove that she's not a threat and tries to make friends. Another could be her discovery of new powers and how she uses them for good. Maybe she teams up with other characters to fight a common enemy.
One plot idea could be that Ash and Android 21 team up to fight a common enemy in a tournament. Another could be a story where Android 21 tries to understand human emotions through Ash's experiences. Or perhaps a plot where they travel through different dimensions together.
One idea could be that the OC is a scientist who is intrigued by Android 18's unique blend of power and humanity. He tries to find a way to make her more 'human' in terms of emotions, leading to a series of adventures where they face various threats together. Another could be that the OC is a time - traveler who accidentally lands in the Dragon Ball universe and forms an alliance with Android 18 to prevent a catastrophic event in the future. And a third idea might be that the OC is a martial artist who challenges Android 18 to a series of battles, which gradually turn into a friendship and then something more romantic as they learn about each other's values.
One idea could be that Android 21's unique powers attract the attention of Naruto. He might see her as a new kind of challenge. Maybe she has some sort of energy that is similar to chakra in a way. And Naruto tries to understand her and her powers, leading to an unexpected friendship.
Maybe in the fanfiction, Android 18 gets injured while protecting Gohan from a new powerful enemy. Gohan then takes care of her and in that time of vulnerability, they realize their feelings for each other. This could lead to them exploring a relationship while also trying to figure out how to defeat the enemy that still lurks.
One idea could be a character from the Enclave finding themselves in a situation where they have to team up with a group from the Fallout world they usually oppose. Maybe a lone Enclave soldier gets separated from his unit and is forced to rely on a group of wastelanders to survive. This could lead to a lot of character development as he sees a different side of the world outside the Enclave's ideology.
One plot idea could be Android 17 in a post - apocalyptic world, using his powers to protect the few remaining humans from other threats. Another could be Android 17 getting transported to an alternate universe where he has to team up with his evil self to defeat a greater enemy. Maybe he could also have a story where he tries to integrate into human society and discovers emotions he didn't know he had.