Well, it would change the story a great deal. Firstly, Harry would have a more stable home environment with Severus as his guardian. He wouldn't be so alone at the Dursleys'. Draco, on the other hand, might learn different values from Severus rather than just pure - blood supremacy from his family. And Hermione would have an adult figure in the wizarding world who truly understood her thirst for knowledge. The whole dynamic between the three of them would shift from rivalry to more of a sibling - like relationship under Severus' care.
They'd probably have a very interesting and complex family dynamic. Severus would be the strict but fair father figure. Harry would be the one with a lot of pent - up emotions from his past, but also a strong sense of justice. Draco would be the one trying to find his place between his old pure - blood values and the new values Severus would teach. Hermione would be the voice of reason and the one always pushing for knowledge. There would be a lot of bickering at first, especially between Harry and Draco, but over time they would grow to care for each other like siblings.
In this scenario, Severus' character experiences a significant transformation. He has to shed his cold exterior to a large extent. He becomes a father figure, which means he has to be there for the boys emotionally. He starts to show emotions more openly, whether it's pride when they do well in their studies or worry when they are in danger. His sense of loyalty also shifts. Before, it was mainly towards certain people or causes, but now his loyalty is first and foremost to Harry and Draco. This new - found loyalty makes him a more rounded and likable character in the fanfiction, as he is no longer just the brooding potions master but a loving and dedicated father.
In some of these fanfictions, an interesting plot development could be Severus having to deal with the boys' different personalities. Harry, being brave and impulsive, while Draco is more reserved and proud. Severus has to find a way to balance their needs. For example, when it comes to their magical education, he might have to teach them in different ways. He could also have to handle the outside world's reaction to him adopting both a former 'enemy' (Harry) and a pure - blood Slytherin (Draco). Another aspect could be how the boys form a bond with each other through Severus' influence. They might start to see past their initial differences and become like brothers, which changes the whole dynamic of the wizarding world they live in.
Maybe Draco discovers that Severus is being framed for a crime he didn't commit. He then approaches Hermione for help as she is known for her intelligence and problem - solving skills. Together, they start to gather evidence to clear Severus's name and also protect him from those who want to harm him.
One possibility could be that Draco and Hermione realize Severus is in danger due to some dark magic plot. They might put aside their differences because they both understand Severus's importance. Hermione's vast knowledge of magic combined with Draco's insider knowledge of the Death Eaters' ways could lead to a unique rescue plan.
His personality also becomes more open. He has to communicate with Harry in a more honest and open way. He can't keep all his thoughts and feelings bottled up like he used to. He has to share his wisdom and experiences with Harry, which in turn makes him more approachable and less of the brooding, closed - off person he was before in the official story.
The two main characters are obviously Harry and Severus. Harry, the hero of the wizarding world, who has a big heart and a sense of justice. And Severus, the complex and often misunderstood former Death Eater and potions master.
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.