A powerful wizard or sorcerer who wants to use Goku's daughter's pure Saiyan energy for some dark magic. He could be creating chaos on Earth, and she has to stop him before he drains all her energy. This antagonist would be a departure from the typical physical - strength - based villains in Dragon Ball, adding a new dimension to the fanfiction.
One possible fanfiction idea could be that Goku's daughter has unique powers that are a combination of his Saiyan strength and something new, like the ability to control time on a small scale. She might struggle to fit in with the other Z - fighters at first because they see her as just Goku's daughter, but she proves herself by facing a new, powerful enemy that the others can't defeat easily.
She could have unique powers like Goku but with a different twist. Maybe she has a special ki ability that no one else in the Dragon Ball universe has. For example, she could control the elements using her ki.
In a fanfiction, Goku's daughter might inherit a special power from her mother (who could be an original character in the fanfiction). This power could be related to healing or some kind of spiritual energy. She uses this power to heal the injured during a big battle against a powerful villain. This makes her an important asset to the Z - fighters, not just because of her Saiyan heritage from Goku, but also because of her unique ability.
One interesting plot could be Goku's daughter having to train in a new dimension. She discovers unique powers there and has to use them to save the Earth from a new threat. This would be a great way to explore new aspects of the Dragon Ball Z universe while also focusing on a new character, Goku's daughter.
Goku's understanding of different universes and races could be deepened in fanfiction. Since Dragon Ball Super introduced the multiverse concept, fanfiction could explore how Goku interacts with and learns from beings from other universes, making him more worldly and less naive in some ways.
Maybe an evil Namekian. Namekians have unique powers, and an evil one could use his abilities to manipulate the life energy in a way that creates zombies. He might be jealous of the Saiyans' power and wants to use the zombies to wipe them out. He could also be in cahoots with some of the other villains from the Dragon Ball series, like Cell. The heroes would have to not only fight the zombies but also figure out this Namekian's motives and how to stop him.
Her character could be developed by giving her unique fighting styles. For example, she could combine Goku's martial arts with some new techniques she invents. This would show her creativity and independence.
Perhaps a fanfiction where Vados senses a new threat approaching the universe. She enlists Goku to help her find a way to stop it. Along the way, they encounter strange new races and powers. Goku's simple but powerful approach to fighting combined with Vados' wisdom would make for an interesting dynamic as they search for a solution to this impending doom.
In a fanfiction, Goku might be weakened due to a curse. He loses his Super Saiyan powers and has to start from scratch. This could lead to a great character - development arc. He might discover new ways to fight without relying on brute strength, like using his wits and strategic thinking more, and also learning new martial arts techniques from his human friends.
Yes, there are quite a few. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Dragon Ball Goku's sister' and you'll find a variety of stories, some focusing on her training, others on her relationship with Goku and the rest of the Dragon Ball gang.