Since we don't have more information about this 'a snowball's chance in hell fanfiction', the main characters could be anyone. It might be a hero - type character who is facing insurmountable odds. Or perhaps it's a group of misfits who are together in this 'hellish' situation. They could be from different backgrounds, but are united by the common theme of trying to survive or achieve something in this highly improbable setting.
One interesting concept could be the exploration of impossible odds, just like the phrase'snowball's chance in hell' implies. The fanfiction might play with the idea of characters defying extreme improbability.
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.
Often, it could be the underdog characters from the original work. The ones who didn't get a fair shot in the original story.
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 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.
In this fanfiction, Harry Potter is the obvious main character as the story seems to revolve around him getting a second chance. Besides him, characters like Ginny Weasley could be important. If the story is about changing relationships, Ginny's relationship with Harry could be explored in a new light. Also, characters from the Ministry of Magic might be involved if Harry's second chance has political implications in the wizarding world.
Obviously Naruto would be one of the main characters since it's in the title. There might also be some characters from the Naruto universe like Sasuke. Sasuke could be involved in Naruto's journey to hell, either as an ally or an adversary. And perhaps some original characters created for the fanfiction that are specific to the hellish setting, like powerful demons or other rulers in hell.
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.
Perhaps Hermione could be one of the main characters in this fanfiction. She's a very strong and intelligent character, and it would be interesting to see her in a love - centered story. It could be about her falling in love with a mysterious new wizard who has some unique magical abilities.