Perhaps a new Saiyan from Planet Vegeta who was sent away as a baby before its destruction and now returns to Earth in the next generation. He could bring with him a different perspective on Saiyan culture and power. He might have been trained in a different way on the planet he grew up on, and his arrival could stir things up among the Earth - based Saiyans.
One idea could be the children of the Z - fighters facing a new, powerful and mysterious enemy from a different dimension. This enemy could have unique powers that the new generation has to figure out how to combat.
The main characters for Dragon Ball Z Gine fanfiction would be Gine first and foremost. Gohan, being Goku's son, could play an important role. Videl, as Gohan's love interest, could also be in the story. Trunks, whether from the future or the present, is another option. And of course, Master Roshi. He has a vast knowledge of martial arts and could be a mentor - like figure to Gine or someone she has to interact with in the story.
Super 17 himself could be the main antagonist if the fanfiction focuses on his corruption. He could be consumed by power and decide to rule over the universe. His already - formidable powers would make him a terrifying opponent for the Z - fighters, and they would have to find new ways to stop him without relying too much on their usual techniques.
Goku could be an interesting main character in a vampire fanfiction. Imagine Goku having to deal with the newfound thirst for blood while still trying to protect the Earth. His strong moral compass would be put to the test as he battles against his vampire instincts.
Goku would be a great main character for a Dragon Ball Z haircut fanfiction. His wild hair is such an iconic part of his look. You could write about how a new adventure changes his view on his hair and he decides to get a different style. Maybe he wants to look more like the people on a new planet he visits.
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.
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, 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.
It could be Orochimaru. In this fanfiction, Orochimaru might hear rumors about the Dragon Balls and their power to grant wishes. Since he is always seeking power and immortality, he decides to hunt for the Dragon Balls. However, because Naruto is so focused on his own battles against Orochimaru in the ninja world, he fails to realize that Orochimaru's new goal involves the Dragon Balls and the entire Dragon Ball Z universe could be at risk.
Maybe an evil scientist who has created a new form of bio - weaponry. He targets Gohan and Lime because they pose a threat to his plans of world domination. His creations could have abilities that are a challenge for Gohan and Lime to counter.