A really interesting plot twist could be that Sirius Black isn't really Sirius Black at all. It could be an imposter, like a Death Eater using Polyjuice Potion. The real Sirius could be imprisoned somewhere. The imposter kicks out the Order as part of Voldemort's strategy to weaken the resistance against the Dark Lord.
Perhaps the plot twist is that Sirius Black kicking out the Order is all part of a grand plan to protect them. He might have received a prophecy or some sort of warning that the Order's current location or actions are putting them in great danger. So, in a rather extreme move, he kicks them out to force them to scatter and thus be less of an easy target for Voldemort and his followers.
One possible plot twist could be that Sirius Black has a hidden motive for kicking out the Order. Maybe he discovers a traitor within the Order and by kicking them all out, he forces the real traitor to expose themselves in order to rejoin. Another could be that Sirius is being influenced by a dark force that he's unaware of, which is making him act out of character. For example, it could be a spell that was cast on him by a Death Eater.
In 'James Sirius Potter: The Order's Deception', there's a huge plot twist where the Order's leadership has been infiltrated by a dark wizard in disguise. James has to figure out who it is before it's too late. The way the author slowly reveals the clues makes it a very engaging read.
One possible plot twist could be that Sirius Black, being a wanted man, has to constantly move with Harry to avoid capture. This would make Harry's childhood very different from the one at the Dursleys. He'd grow up learning about the wizarding world on the run, perhaps meeting some unique characters in hiding places.
One possible plot twist could be that Sirius Black has a noble reason for 'kidnapping' Harry. Maybe he discovers a plot against Harry within Hogwarts that the school's authorities are ignoring. So, he takes Harry away to protect him. For example, he might find out that a certain teacher is under the influence of Voldemort and plans to harm Harry. Another twist could be that Sirius Black is being controlled by a dark magic spell and doesn't really mean to harm Harry. It could be a setup by Voldemort's followers to make Sirius look bad. Also, it could turn out that the 'kidnapping' is part of a secret training plan for Harry. Sirius wants to train Harry in some special magic skills away from the prying eyes of Hogwarts.
In fanfiction about Sirius Black in the Order of the Phoenix, one common theme is his struggle with the restrictions placed on him. He's a free spirit, but the Order has its rules. Some fanfictions might depict him chafing against these rules, while still trying to do what's right for the cause. His connection to the Black family legacy is also an area of exploration. How does he reconcile his hatred for his family's dark past with the fact that he's still a Black? Fanfiction can provide various interpretations, showing his internal conflict and how he overcomes it to be a valuable member of the Order.
One of my favorite scenes in Sirius Black Order of the Phoenix fanfiction is when he finally gets to have a proper heart - to - heart with Harry. It shows his vulnerability and how much he cares for Harry. Another great scene is when he uses his animagus form to outwit the Death Eaters and save the day. It really showcases his cleverness and his willingness to put himself in danger for the greater good.
In a fanfiction where Sirius Black adopts Hermione Granger, a plot twist could be that the magical community doesn't accept this adoption easily. There could be a lot of opposition from different factions. Some might think that Sirius, with his past as a supposed criminal, isn't a suitable guardian. Meanwhile, Hermione could be facing problems at Hogwarts because of this new family situation. However, through their bond, Sirius and Hermione could overcome these difficulties and in the process, Hermione could learn some long - lost family magic from Sirius, which gives her an edge in dealing with the challenges that come their way.
Perhaps the twist is that Sirius saves Draco, but in doing so, he sacrifices his own freedom. And then Draco has to figure out a way to return the favor and free Sirius. Or it could be that the reason Sirius saves Draco is not what it seems at first. Instead of pure altruism, he has a hidden motive related to his own past or a prophecy that involves Draco.
Well, a common plot twist is the idea of soulmates. In some fanfictions, it is discovered that Hermione and Sirius are soulmates across different timelines or dimensions. This leads to their kiss as they are drawn to each other on a deeper level. Another plot twist could be a misinterpretation of prophecies. The fanfiction might have a prophecy that seems to point to Hermione and Sirius being together, which causes them to explore their relationship and end up kissing. Also, the idea of a parallel universe where the rules of the original story are different can lead to them kissing, such as a universe where age differences don't matter as much in relationships.
A plot twist could be that Hermione is initially oblivious to Sirius' feelings. She's so focused on her studies and the fight against Voldemort that she doesn't notice the signs. But then, one day, she accidentally stumbles upon a letter Sirius was writing about his feelings for her. This throws her into a state of confusion as she never expected such a thing from him.
In some fanfics, a plot twist could be that Severus was actually protecting Hermione all along under Dumbledore's secret orders, and Sirius discovers this and has to re - evaluate his hatred for Severus. This completely changes the dynamic between the three.