Draco could be seen as the main antagonist. Since the title implies that Harry breaks his face, it's likely that Draco did something to provoke Harry in the first place. Maybe he was bullying Harry's friends or being his usual snobbish self towards Harry.
Well, it could be a story where there's a huge fight between Harry and Draco. Maybe Draco says or does something really offensive to Harry or his friends, which makes Harry lose his temper and accidentally break Draco's face. Then it might deal with the consequences, like how the Hogwarts staff reacts, and how Harry feels guilty about it afterwards.
Draco might become withdrawn. He could isolate himself from his friends and spend more time alone in the Slytherin common room, brooding over what Harry has done to him.
One possibility is that Draco has been taunting Harry about his family or friends again. Harry, pushed to his limit, loses his temper and punches Draco. This could lead to a big conflict within Hogwarts, with students taking sides either for Harry's act of standing up against Draco's constant bullying or against Harry for using violence.
It would definitely strain their relationship further. Draco would likely be extremely angry and vengeful. He might plot all kinds of schemes to get back at Harry, from simple pranks to more dangerous magical attacks. Harry, on the other hand, might initially feel a sense of satisfaction for standing up to Draco, but later he could start to regret his actions when he realizes that violence doesn't solve anything and has only made an already bad situation worse.
There's also 'Fractured Bonds'. In this fanfiction, Draco breaks Harry's heart by siding with the wrong crowd at a crucial moment. It could be that Draco is pressured into doing so, but the result is Harry feeling betrayed and his heartbroken. This often leads to a downward spiral for Harry as he tries to come to terms with what has happened.
At first, Draco might be in shock. He didn't expect Harry to break up with him at such a crucial time. Then he could try to reason with Harry, begging him to stay for the sake of their unborn child. When Harry refuses, Draco could turn to his own family for support and close himself off from the outside world for a while.
One possible reason could be a misunderstanding. Maybe Harry misinterprets Draco's actions or words in a way that makes him do something that breaks Draco's heart. For example, if Draco is secretly trying to help Harry but Harry thinks he has some malicious intent and publicly shames him.
One challenge is dealing with their different personalities. Harry is brave and headstrong, while Draco is proud and used to privilege. Snape has to find a way to discipline them without causing resentment. Another challenge is the expectations of the wizarding community. People have certain views of Harry as the 'Chosen One' and Draco as a Malfoy. Snape has to protect them from those outside pressures.