One possible plotline could be that Harry discovers a powerful time - traveling spell in his third year at Hogwarts. He decides to use it to go back in time and save his parents. Along the way, he has to face challenges like avoiding detection by the time - turner regulations and dealing with dark wizards who are also lurking in the past.
Maybe in the fanfic, Harry finds a magical artifact that gives him the ability to change the past. He uses it to go back to the night Voldemort attacked his parents. He then uses his advanced magical skills, which he learned from his years at Hogwarts, to fight off Voldemort and save his parents. He might also have to enlist the help of some unexpected allies from that time period.
Obviously, Harry Potter is the main character as he is the one trying to save his parents. His parents, James and Lily Potter, would also be important characters even though they are the ones being saved. There might also be other characters like Dumbledore. Dumbledore could play a role in guiding Harry in his mission, either by providing advice or by giving him some magical tool or knowledge that helps him in his quest to save his parents.
One plotline could be Harry finding a new family among his friends at Hogwarts. They become his support system and help him grow in ways his biological parents couldn't. Another could be Harry's journey to discover why his parents disowned him and uncovering some dark family secret along the way.
In this fanfic, a really cool plot could be that Harry has been secretly communicating with other wizards outside of his family through a magical object. They've been planning his escape for a long time. They send him a special charmed item, like a bracelet that can turn him invisible at will. He uses this to his advantage and slips away from his family during a chaotic moment, like when they're having a big argument. Then, he follows a series of clues left by his new friends to reach a safe haven, a hidden magical community where he can finally be free and learn more about his true potential.
A common plotline is that Daphne saves Harry from a dark magic attack. For example, she might notice a cursed object meant for Harry and intervenes just in time.
A plotline could involve Petunia having a change of heart due to a magical event. For instance, she might accidentally stumble upon some of Harry's magic or receive a message from the wizarding world about his importance. This could lead her to stand up against Vernon and protect Harry. She could then start to teach Harry about his mother's side of the family and the magic that runs in his veins in a more positive way than the Dursleys ever did. And perhaps she even helps him develop his magic skills secretly before he goes to Hogwarts.
A third plotline might be that Harry is abandoned in a magical but hidden place. There, he is raised by magical creatures, say, a group of centaurs. They teach him their own brand of magic and wisdom. When he finally enters the wizarding world proper, he has a very different perspective on magic compared to the other students, which causes both admiration and some envy among his peers.
A plotline could be that Harry, after being dumped by his parents, has a very different personality. He's more withdrawn and distrustful. Sirius has to work hard to break through those walls. They could travel to different magical places to find a way to reverse what Harry's parents did or to find out the real reason behind it. Along the way, they meet other characters who either help or hinder their journey, and this all shapes their relationship and Harry's growth as a person.
One possible plotline could be Harry growing up in an orphanage where he discovers some latent magic powers on his own before getting his Hogwarts letter. He might face more difficulties and prejudices from the muggles around him, which could make his entry into the wizarding world even more of a relief.
One possible plotline could be Harry trying to find out why his parents forgot him. Maybe he discovers a dark spell was cast on them. Another could be his journey to regain their memories, facing many challenges along the way.
In certain 'Harry Potter parents in stasis fanfiction', there could be a plot where the stasis was a form of protection. When they awake, they discover that there are still some Death Eaters who are after them. They then use their old skills and knowledge, combined with the new things they learn about the current wizarding world, to fight back. They also have an emotional journey as they reconnect with Harry and other friends and family they left behind.