One idea could be that Snape discovers a new dark magic plot against Harry early on. He then uses his position as a double agent to mislead the dark wizards while secretly training Harry in advanced defensive magic. For example, he takes Harry to a hidden chamber in Hogwarts at night to practice spells that are not taught in regular classes.
One interesting plot idea could be that Snape raises Harry with a strict but fair hand. He teaches Harry about the Dark Arts not to encourage him to use them, but to make him fully aware of the dangers. Harry might grow up with a more nuanced view of Snape from the start, and their relationship could be a mix of father - son dynamic and teacher - student. For example, Snape could train Harry in potions from a very young age, leading Harry to become a master of potions himself much earlier than in the original story.
One idea could be that during the duel, Harry discovers a hidden spell that Snape was about to use against Voldemort. So instead of a full - on fight, they end up in a truce to discuss this new revelation. This could lead to a whole new understanding between them.
One plot idea could be Harry struggling to fit in at first in Slytherin because of the house's reputation for dark magic, but then he discovers the true values of loyalty and cunning within it. He could form an unlikely friendship with a pure - blood Slytherin who helps him see that not all in Slytherin are bad. Another idea is that Harry uses his newfound Slytherin traits to outwit Voldemort in a different way than in the original story, perhaps by using more stealth and manipulation.
Perhaps Harry is adopted by a Muggle family who are aware of the wizarding world. They could be a family of scientists who study magic from a scientific perspective. Harry would grow up with a unique understanding of magic, seeing it both as a wonder of the wizarding world and something that can be analyzed. This might make his time at Hogwarts very interesting as he tries to blend these two views of magic.
One plot idea could be Snape teaching Harry some long - forgotten dark arts defenses that were once known only to the founders. This could put Harry in a unique position in his battles against Voldemort. Another idea is that Snape reveals to Harry some secrets about his mother, Lily, that help Harry understand his family better and give him more motivation in his fight. Also, Snape could mentor Harry in potions in a more personal way, sharing his own experiences and insights which could lead to Harry creating new and powerful potions to use against the dark forces.
One idea could be that Harry discovers the truth about his parentage early on. This changes his relationship with Dumbledore as he realizes Dumbledore had been keeping this big secret from him. He might also struggle with his identity, feeling torn between the Gryffindor ideals he was raised with and the Slytherin traits he might inherit from Snape.
A plot could be where a rare and endangered creature chooses Harry and Snape as its protectors. They must travel to different magical locations to find a safe haven for it. This would force them to interact and rely on each other's skills.
One idea could be that Harry, as Voldemort's soulmate, tries to use their connection to redeem Voldemort. He might show Voldemort memories of love and kindness in an attempt to change his dark ways.
One idea could be that Harry discovers his connection to Morgana gradually. Maybe he starts having strange dreams related to her. As he digs deeper, he finds that some of his powers are actually linked to Morgana's magic, which sets him on a journey to understand more about his heritage and how it affects his role in the wizarding world.
Perhaps the pregnancy causes Angelina to discover new magical abilities within herself. As she is carrying new life, she starts to have prophetic dreams about the future of the wizarding world. This could lead her on a journey to find out more about these dreams and how they might be related to her unborn child.