The plot could take a completely different turn. For example, if Lucius decides to expose this encounter, it could cause a huge scandal in the wizarding world. Harry's friends might be shocked and some might even turn against him. Draco, on the other hand, would have to deal with his family's reaction, which could be very harsh. It could also lead to a rift between the two boys if they have to side with their respective sides in the face of this situation.
If Lucius walks in, the power dynamic between Harry and Draco would likely shift. They would no longer be in their own little world and would have to deal with an adult authority figure. Draco might become more submissive to his father's commands, and Harry would have to face the wrath of Lucius, which could put him in a very difficult situation.
In some fanfictions, Lucius walking in could be the catalyst for a change in the relationship between the three characters. Maybe Lucius realizes that the enmity between Harry and Draco is pointless. He could, in a very un - Lucius - like way, encourage them to get along. This could be part of a larger plot where Lucius has a change of heart and starts to question the long - held beliefs of the pure - blood supremacy, seeing that Harry and Draco's relationship could be a symbol of a new, more inclusive wizarding world.
If they join the Order, their relationship could transform in many ways. Lucius, as a father, might feel a sense of pride in Draco for making a different choice. Draco, in turn, could see his father in a new light, as someone who is willing to change. However, there could also be some strain. Lucius has a lot of baggage from his past deeds, and Draco might struggle with how to deal with that while still supporting his father in the new environment of the Order.
The plot could take a turn towards drama. Ron's reaction could create a rift between him and Harry, forcing Harry to choose between his old friend and this new - found connection with Draco. This would add a lot of tension to the story.
Another possibility is that Lucius could be initially confused. He might not understand what he's seeing at first. After all, Harry and Draco are supposed to be enemies in the general wizarding world view. He could stand there for a moment, trying to process what's going on before taking any action.
His arrogance might decrease. Living as a Muggle, he'd face situations where his wizarding status doesn't matter. He might become more humble as he realizes he can't always rely on magic to solve problems.
In this scenario, it could also lead to a major shift in the relationships within the fanfiction. Hermione might feel betrayed if Draco doesn't stand up to his father. And Draco could be forced to make a difficult choice between his family's expectations and his feelings for Hermione. Lucius's interference could also bring in other elements of the wizarding world, like the Ministry or other pure - blood families getting involved depending on how the situation unfolds.
Their relationship would likely become more collaborative. They would have to communicate regularly about Teddy's needs, like his schoolwork, his magical training, and his emotional well - being. This would force them to put aside their past grudges and start seeing each other in a new light.
Draco's relationship with Hermione could change drastically. He might start to feel sympathy for her, which could turn into a reluctant friendship. Lucius, however, would be more resistant to any positive change. But as Hermione shows her strength even while injured, he might develop a begrudging respect. Overall, Draco could become more of an ally to Hermione, while Lucius would be in a more complex situation of having to re - evaluate his views due to Draco's new attitude.
This incident could also introduce new storylines. Maybe Sirius tries to keep Harry and Draco apart, which then leads to a series of events where they have to find ways to be together or communicate secretly. Or perhaps Sirius' reaction causes other characters to get involved, expanding the scope of the fanfiction plot.
Lucius may become more conflicted. He starts off as a loyal Death Eater, but having Harry in his care makes him question his loyalty to Voldemort. He might start to see the humanity in Harry and that could soften his cold - hearted nature. For example, he may find himself protecting Harry from other Death Eaters.
In this fanfiction, Harry might also save Draco by appealing to Lucius' sense of self - preservation or family. He could threaten to expose some of Lucius' secrets or illegal activities if he doesn't let Draco go. Or, if there is some underlying family drama involved, Harry could use that to his advantage. For instance, if there's a feud within the Malfoy family that Harry is aware of, he could manipulate the situation in a way that makes Lucius back off and leave Draco unharmed.