Often, it could be the underdog characters from the original work. The ones who didn't get a fair shot in the original story.
Naruto is likely to still be one of the main characters. His journey and growth are central to the Naruto universe. Sasuke would probably also be a main character as his relationship with Naruto is so important. Sakura might also be there, as she was an important part of the original team. And Kakashi could be a main character as well, being their sensei.
Well, the central character is always Regulus Black. In many of these fanfictions, his family members play important roles. Bellatrix Lestrange might be included to show the contrast between the two Blacks. And of course, Dumbledore could be in the story, as he is a key figure in the wizarding world. He might be the one to give Regulus the second chance or at least be aware of it. Additionally, some fanfictions might introduce original characters who help Regulus in his second - chance journey.
Often, it's the ex - lovers. They are the central figures as the whole story is about their second chance at love.
Typically, the children of the original characters like Harry Potter, Hermione Granger, and Ron Weasley are main characters. So, you might have Albus Potter, Rose Granger - Weasley, etc. They inherit the legacies of their parents and often have to live up to great expectations.
It's hard to say without actually reading the fanfiction. It could be the underdog character who is seeking a new start in life. Maybe it's a young adventurer who has been down on their luck and this chance to change fate is what sets them on a new path.
In a 'Twilight' fanfiction titled 'take that chance', it might introduce a new love interest for one of the main characters. So, aside from Bella, Edward, and Jacob, there could be a new vampire or human character who becomes a main focus as they are the one taking the chance. For example, a new vampire arriving in Forks and taking a chance on fitting into the Cullen - human - werewolf dynamic.
It could be Goku. Since he's the main protagonist in DBZ, often in fanfictions he's central even when the theme is about a second chance. He might be the one getting the second chance or be involved in someone else's second - chance story.
Naruto is often still a main focus. His growth and new adventures in this 'new world' are usually explored. Sasuke is also likely to be a key character, as his relationship with Naruto and his own journey of self - discovery are important aspects. Sakura might also be a central character, especially if the story involves medical ninja aspects or her relationship with Sasuke and Naruto.
I'm not sure specifically as I haven't read the exact novel. But usually in a love - themed series novel, there would be the two main characters who are at the center of the love story. There might also be friends or family members of the main characters who play important roles in either facilitating or creating obstacles for their relationship.
Often, there are kids as main characters. They go on all sorts of adventures, like exploring a mysterious forest or an old castle.