A fanfiction might explore the idea of Bardock seeing Goku's life through a time - space rift. He witnesses all of Goku's battles and the friends he makes. Bardock is filled with pride but also realizes that Goku has a different path than the traditional Saiyan way. This makes him question the Saiyan culture he once knew. Meanwhile, Goku starts to have strange dreams about his father, which leads him to search for more about his Saiyan heritage.
In some fanfictions, Goku's death might be a sacrifice to save the universe. He could be facing an enemy so powerful that the only way to stop it is by giving up his own life. This would be a very emotional moment for his friends and family, like Chi - Chi and Gohan.
One interesting Dragon Ball Z Kai fan fic could be one that focuses on an alternate timeline where Goku never hit his head as a baby. This would change his personality and the entire course of the story. Another could be a story centered around Vegeta's training in a new, uncharted planet, exploring his growth without the constant presence of Goku. There are also fan fics that might explore the backstories of minor characters like Yamcha or Krillin in more depth, which can be really fascinating.
One possible plot for this fanfiction could be that Bardock's return is not a simple one. He might be brought back by some sort of time - travel or resurrection magic. His return could stir up old rivalries among the Saiyans. And as he tries to fit into the current Dragon Ball Z universe, he discovers a conspiracy that threatens not only the Saiyans but the entire universe. So, he has to fight against this conspiracy while also trying to reconnect with his son Goku in a more meaningful way.
The main characters often include Dark Goku and the Z - fighters. Among the Z - fighters, Piccolo is an important one. His wisdom and combat abilities can be crucial in dealing with Dark Goku. Krillin also has his place. His long - standing friendship with Goku and his own fighting skills add depth to the story. And Master Roshi might also make an appearance, perhaps as an advisor or someone who has some knowledge about an ancient power that could be relevant to the fight against Dark Goku.
One common theme is empowerment. Since Goku is already a very powerful character, in these fanfics where she's female, it often emphasizes her strength even more, showing that gender doesn't limit her abilities.
Vegeta could also work well. Vegeta's pride and determination, along with his powerful fighting skills, would contrast nicely with the Supreme Kai's more reserved and wise nature. They could learn from each other and have some intense rivalries within the story.
Obviously, Bardock himself. And of course, Goku would likely be in it since he's Bardock's son. Vegeta might also show up, as he is an important Saiyan character. There could also be other Saiyans like Nappa or Raditz, especially if the story is about Bardock trying to reform or reunite the Saiyan race.