It could be a side character from the original work who didn't get much spotlight. Giving them a chance to live could provide an opportunity to explore their backstory and develop their character further. For instance, a minor character who was killed off early in the story could now become the hero.
Kagome might be the main character. As she's from the modern world and has a big influence on the events in the feudal era, she could be the one who gets a chance to change the fates of everyone. For example, she could use her modern knowledge to change the course of battles or relationships in the story.
It could be a Pokemon itself. For example, a Pikachu that has been separated from its original trainer and has to fend for itself in the wild. But then it meets a new friend, either a human or another Pokemon, who helps it start over. This Pikachu could be the focus of the story as it overcomes its past traumas and looks forward to a new life.
It could be an older trainer who has been through a lot in the Pokemon world and is now getting a second chance. Maybe someone who was once a gym leader but lost their position due to some scandal and now wants to prove themselves again.
Shido could be an obvious choice as the main character. Since he has a special connection with the spirits in the original story, his time - traveling adventures could add new dimensions to his relationships with them. He could go back in time to understand the origins of the spirits better and find new ways to save them.
Darth Vader could be the main character in a second - chance fanfic. He could be brought back to life and given the chance to fully embrace the light side from the start. He could use his knowledge of the dark side to help the Jedi fight against a new and powerful evil in the galaxy.
It could still be Voldemort, but in a different form. Maybe he has a new power or a different plan this time. Since it's a new chance, he might have found a way to come back even stronger and more devious than before.
It could still be Voldemort. Even though Harry has a second chance, Voldemort is still a major threat in the wizarding world. His presence and his Death Eaters would continue to be the main source of danger, and Harry would need to use his second chance wisely to defeat him once and for all.
It could be Harry Potter himself. Given all the hardships he faced in the original series, a second chance would open up a whole new world of possibilities for him. He could use this chance to prevent some of the tragedies that occurred, like the deaths of his friends or mentors.
It could be a new dark wizard or witch who rises to power. Maybe someone who was once in Voldemort's shadow but now sees an opportunity with Voldemort gone (in this new fanfiction scenario where Snape has a second chance). This character could oppose Snape's attempts to reform the wizarding world and bring about a new era of peace and magic.
Perhaps Orochimaru. He might see Sakura's distraction from missing her chance as an opportunity to target Naruto or gain some advantage in his evil plans. He could manipulate the situation, making it even more difficult for Sakura to fix what she has missed.