Severus would probably use his knowledge of dark magic and potions to protect Harry. He could create protective spells and potions that would shield Harry from any immediate threats. For example, he might brew a potion that makes Harry invisible to dark wizards for a short period of time.
Well, in such a fanfiction, one possibility is that Severus could use this opportunity to finally reveal some long - held secrets to Harry. Maybe he tells Harry about his true loyalties all along and how he was actually protecting him in a very convoluted way. Another thing that might occur is that on the way to get the prophecy, they could face some dark magic threats that Severus has to use his skills to fend off while also keeping Harry safe.
At first, it would be extremely strained. Severus has a history of being at odds with Harry. But with the baby in the picture, they would be forced to interact. Harry might see a different side of Severus as he tries to be a father. Severus, on the other hand, would have to deal with his own feelings towards Harry while also caring for the baby. Over time, they could develop a bond based on their shared responsibility and the love for their child. This would be a huge transformation from their previous relationship which was full of mistrust and dislike.
At first, their relationship would be full of hostility. Harry would be angry about being forced into this marriage with Severus, who he has had a complex history with. Severus, on the other hand, might be indifferent or even disdainful.
Severus would likely become more protective. His personality would shift from being mainly focused on his own grudges and potions work to being centered around Harry's well - being. He might also become a bit more open - minded as he has to consider Harry's needs and wants, which could be different from his own. For instance, he may have to tolerate Harry's love for flying, even though he himself may not be into it. And over time, he might develop a sense of humor as he deals with Harry's childish antics.
He would become more nurturing. Usually, Severus is seen as cold and aloof, but with Harry in his care, he'd have to show more warmth. He might still be strict, but it would be out of love for Harry's well - being rather than just being his usual harsh self.
At first, Severus would be exasperated by Harry's mischievousness. He might deduct points from an imaginary house just like he does in Hogwarts. But then, he'd remember that Harry is just a child and doesn't fully understand the consequences. He could then try to use positive reinforcement. For example, if Harry behaves well for a certain amount of time, he gets to learn a new spell or potion ingredient. This way, Severus can gradually shape Harry's behavior while also building a closer relationship with him.
At first, Severus would probably be very suspicious of Harry. After all, Harry is an unknown figure suddenly appearing. But Harry, being the friendly person he is, would try to break the ice. He might start by complimenting Severus on his potion - making skills, which Severus is already quite good at even as a young boy. Their interaction would be a bit tense initially, but as they start to understand each other more, they could become friends or at least allies.
Harry might develop from being resistant to the marriage to being more open - minded. He realizes that Severus is not just the strict teacher he once thought. For Severus, he may start to soften his tough exterior. He begins to see Harry not as James Potter's son but as an individual with his own merits.
At first, it's likely to be full of tension. Harry may be hesitant and even a bit resentful about the marriage, while Severus is his usual cold self. But as time passes, they start to see each other's good sides. Harry realizes Severus's intelligence and Severus sees Harry's courage in a new light.
It could lead to a big change. Maybe Harry and Dudley would learn more about Severus and develop a different relationship with him.