Trunks could be a key focus. Since he already had experiences of time - traveling and trying to change the future in the original story, in a second chance fanfiction, he could use his knowledge in a different way. He might be able to prevent the disasters that led him to travel back in time in the first place, which would completely rewrite the future of the Dragon Ball Z world.
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.
Vegeta also stands out. Given his complex character arc in the original series, a second chance could be used to explore his past more deeply. He could make different choices regarding his rivalry with Goku and his loyalty to the Saiyan race. Maybe he would be more open to working with the Z - fighters from the start.