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.
The main character is Bardock, so he's a given. Goku is bound to be in it. But also, characters like Krillin could be there. Krillin is Goku's best friend and his presence could add some comedic relief or be part of a side plot. Master Roshi might also make an appearance. He has a lot of knowledge and wisdom, and could be a guiding figure for Bardock as he tries to understand the current state of the Dragon Ball Z world. And we can't forget Frieza. If Bardock returns, there could be some unfinished business between them, which would make for an exciting plot point.
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.
One great Bardock in Dragon Ball Z fanfiction is often depicted as a fearless and noble Saiyan. He is shown as a warrior who, despite the cruel nature of the Saiyan race at that time, had a sense of honor. In some fanfictions, he manages to break free from the control of Frieza in more creative ways than in the original story, making his character even more complex and interesting.
Goku is a great character to pair with Bardock. Since Goku is his son, it can create some really emotional and action - packed scenes. They can train together or fight side by side against common enemies. Another good option is Vegeta. Their different personalities as Saiyans can lead to interesting interactions. Bardock's old - school Saiyan ways compared to Vegeta's pride can make for some engaging storylines.
Bulma is a likely main character. She's always been part of the group and would bring her intelligence and her gadgets to the beach setting. Gohan too, as he often gets involved in the group's adventures. His kind nature would make him a great character to have around on the beach, helping others and having fun.
Goku is almost surely going to be one of the main characters. He's the face of Dragon Ball after all. Vegeta is also very likely, as his rivalry and friendship with Goku is a central part of the series. And of course, Bulma might be there, she's been important since the very beginning, providing the gang with all kinds of cool gadgets.
Obviously, Bardock is one of the main characters since it's about his rebirth. Goku might also be involved as Bardock is his father. Vegeta could potentially show up too, as he is an important Saiyan in the Dragon Ball Z universe.
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.
A popular version of Bardock in fanfiction is the one who has a premonition of the destruction of the Saiyans much earlier. This Bardock then tries to take actions to prevent it, like gathering other Saiyans who might be sympathetic to his cause. It makes him a hero figure right from the start, rather than just a victim of Frieza's wrath as in the original story.
Goku and Vegeta are likely to be main characters. They are the two most powerful Saiyans and often at the forefront of any major threat in the Dragon Ball Z universe. Also, Piccolo, as one of the smartest and powerful characters, would probably play an important role.
Frieza is an obvious choice. He is the main villain associated with the destruction of Planet Vegeta. Having him in the fanfiction would create a lot of tension. Bardock and Gine could be on a mission to take revenge on Frieza, which would make for a very exciting and action - packed story.