In a 'Draco Malfoy Joins the Order' fanfiction, a great plot twist could be that Draco brings with him a powerful and dangerous magical artifact from his family's vault that has the potential to either save or destroy the Order, depending on how it's used. Also, Draco might be the key to uncovering a mole within the Order. He notices something suspicious about one of the members because of his past experiences with the Death Eaters. This revelation could turn the whole dynamic of the Order upside down and lead to a whole new set of challenges and conflicts.
One big plot twist could be that Draco is actually a triple agent. Everyone in the Order thinks he's on their side, but he's also still somewhat loyal to his family and the Death Eaters, and he's playing both sides against the middle. Another twist could be that Draco discovers a secret about his family's past that makes him question everything he thought he knew, and this discovery is what really cements his loyalty to the Order. For example, he might find out that his family was once blackmailed into serving Voldemort.
A really dramatic plot twist could be that Draco joins the Order, but it turns out to be a trap set by Voldemort. Voldemort wants to use Draco to infiltrate the Order and destroy it from within. However, Draco actually has a change of heart and instead of following Voldemort's plan, he turns against him completely. Another possible twist is that Draco's joining the Order causes a rift within his own family. His parents disown him, but this only makes him more determined to fight for the Order's cause, and he ends up becoming one of their most valuable members.
One possible plot twist could be that Draco has to constantly prove his loyalty to the Order. Since he was a former Death Eater, the members might be skeptical at first. For example, they could give him dangerous tasks to test his commitment. Another twist could be his relationship with Harry Potter within the Order. They might start as reluctant allies but gradually become friends, which would be a huge shift from their school - time enmity.
A great plot twist could be related to their friends. Since they are twins, their friends from Gryffindor and Slytherin are forced to interact more. Hermione and Ron might initially be very skeptical of Draco being Harry's twin. But as they get to know him better through the connection with Harry, they find that there are good sides to Draco that they had never noticed before.
One possible plot twist could be that Draco has a hidden agenda all along. Maybe he's actually trying to protect a particular Horcrux for his own reasons. Another could be that he discovers a new Horcrux that no one else knew about, throwing the whole hunt into chaos.
If Draco joins the Order, his character could experience a huge shift. He'd have to confront his family's beliefs. He might start to see the value in the Order's cause, like fighting against Voldemort's pure - blood supremacy. This could lead to him using his Slytherin cunning in new ways, perhaps by infiltrating Death Eater meetings and passing on information. His relationships with other Order members would be complex. Harry might be initially suspicious but could come to trust him over time. Draco would also have to deal with the internal conflict of betraying his family's long - held values.
One interesting plot twist could be Draco secretly helping Harry Potter in the Triwizard Tournament tasks. Maybe he realizes that there's a greater danger at play that could affect both the wizarding world and his own family, so he decides to put aside their rivalry for a while.
His character would likely become more empathetic. Being around the members of the Order, who fight for good, would expose him to different values. He might start to question the beliefs he grew up with as a Malfoy.
Perhaps voldemort's daughter has some hidden power that Draco discovers. This power could be the key to defeating the remaining dark forces in the wizarding world. However, using this power comes with a great risk, and they have to decide if they are willing to take that chance. It would create a lot of tension and moral dilemmas for both characters.
One possible plot twist could be that Riddle has a secret connection to Hermione's family. This would completely change her perspective on him and how she approaches the situation. Another could be that Draco is actually working undercover for a good cause all along and has been trying to get close to Riddle to expose him. And perhaps Hermione discovers a new form of magic that can only be harnessed with the help of both Draco and Riddle, forcing them into an uneasy cooperation.
Perhaps in the fanfiction, Draco Malfoy turns out to be a double agent. He pretends to be on the side of the Death Eaters like his family, but in reality, he is secretly helping Harry Potter and the Order of the Phoenix. Snape discovers this and has to decide whether to expose Draco or use his secret to their advantage. This would create a lot of tension and unexpected alliances.
One interesting plot twist could be that Fem Harry's presence among Voldemort's ranks causes internal conflict. Some Death Eaters might be distrustful of her because she was once the 'Boy Who Lived' in a female form. Another twist could be that Fem Harry starts to influence Voldemort's ideology in a way he didn't expect. Maybe she makes him question his pure - blood supremacy by showing that her Muggle - born friends were as powerful as any pure - blood wizard. Also, it could be that her joining was part of a long - con plan to bring down Voldemort from the inside, and she gradually starts to turn his own followers against him.