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 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.
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.
Goku is very likely. He's the main character in Dragon Ball Z. If there's a second chance scenario, his decisions and actions would have a massive impact on the story. He could use the second chance to protect his loved ones better or to reach an even higher level of power.
The plot might involve a new threat emerging on the beach. For example, an alien who is attracted to the energy of the Z - fighters shows up. The fighters then have to protect the beachgoers while also trying not to cause too much destruction. Piccolo could use his powers to create a shield around the beach area to protect everyone.
Obviously, Android 21 is one of the main characters. She's the center of this fanfiction. Also, Goku and Vegeta are likely to be involved as they are the mainstays in the Dragon Ball Z universe. They would interact with Android 21 in different ways, like fighting against her or trying to understand her motives.
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.
The main characters in 'Dragon Ball Z' are Goku, Vegeta, Gohan, Piccolo, Krillin, and Bulma. Goku is the protagonist, a Saiyan sent to Earth as a baby. Vegeta is a proud Saiyan prince. Gohan is Goku's son, who also has great power. Piccolo is a Namekian and a former enemy turned ally. Krillin is Goku's best friend, and Bulma is an inventor who is part of the gang from the very beginning.
Goku is one of the main characters. He's a powerful Saiyan always seeking to become stronger.
Goku is, without a doubt, one of the most popular. His constant pursuit of strength and his ability to overcome seemingly impossible odds make him a favorite for fanfiction writers. They can create all kinds of new adventures for him, like facing off against cosmic threats even greater than what he's encountered in the shows.
Obviously Naruto would be one of the main characters as the title implies he is the one being trained. King Kai would also be a major character as he is the trainer. There might also be some cameos or interactions with Dragon Ball Z characters like Goku or Vegeta, since the fanfiction combines the two universes.