It could be that the dark arrow is part of an ancient curse. When Chris is hit, he starts having visions from the past related to the origin of the curse. These visions guide him to find a hidden relic that has the power to break the curse not only for him but also for an entire community that has been suffering from it for ages.
One possible plot twist could be that the dark arrow has some sort of magic that starts to transform Chris into a dark being against his will. As he struggles with this new dark side, his friends or family have to find a way to reverse the process.
Maybe the dark arrow was sent by a wrongdoer who actually targeted the wrong person. Chris, instead of succumbing to the arrow's harm, decides to track down the sender and in the process discovers a conspiracy that was much bigger than he thought. So, he has to fight against the real villains while dealing with the after - effects of the arrow hit.
It could be a dark wizard. Dark wizards are often associated with using dark arrows as weapons in fanfiction. They might have a vendetta against Chris for some reason, like he unknowingly disrupted one of their dark magic experiments.
In some 'naruto green arrow fanfiction', a plot twist might be that Naruto loses his chakra but gains the ability to use Green Arrow's archery skills with a supernatural proficiency. This forces him to completely change his combat approach and rely on a different set of skills, while also dealing with the emotional turmoil of losing his original power.
A possible plot twist could be that Obi - Wan was actually faking being turned to the dark side all along. He was trying to infiltrate the Sith ranks to learn their secrets and stop them from within. He lets Anakin think he has succeeded in turning him, but in reality, he is still a Jedi at heart and has a plan to take down the Sith.
Well, perhaps the Joker could manipulate the legal system in ways no one expects. He might use his charm and chaos - loving nature to turn the trial into a circus.
A plot twist could be that a character from one of the shows, who seems to be a minor or unassuming figure, is actually a powerful being from the future sent to manipulate the events of the crossover. This person has been subtly guiding the actions of the heroes, and when the truth is revealed, it completely changes the heroes' perception of the events they've been through and who they can trust.
A possible plot twist is that Piper's pregnancy affects her powers in unexpected ways. For example, her ability to freeze time might start to act up and freeze random things or people that she doesn't intend to. And perhaps Chris has a connection to an old enemy that resurfaces because of the pregnancy, putting Piper and the baby in danger.
A plot twist could be that they are joined by other Jedi who also believe that the Dark Side has been misjudged. Together, they form a new order that tries to balance the Light and Dark. But this new order is seen as a threat by both the Jedi and the Sith. So Anakin and Obi - Wan have to fight on two fronts, against those who once were their allies and against the true Sith Lords who want to use them for their own evil purposes.
One possible plot twist could be that Dark Naruto tries to change the past to prevent his own dark transformation. He might attempt to save someone important who in the original timeline led him down the dark path. For example, if it was the death of a certain character that made him turn dark, he could go back and try to save that person. This would completely change the future and the Naruto we know from the original story.
A plot twist could be that Loki's dark circles are actually a sign of a hidden power. In the fanfiction, as Loki discovers this, he has to learn to control this new aspect of himself. It could be that the darker the circles, the more powerful he becomes, but he also risks losing his sanity if he doesn't master it.
One interesting plot twist is when the bow and arrow are actually a key to an ancient wizarding prophecy. In a particular fanfic, Harry finds this old, dusty bow in a hidden chamber. Little does he know that it's tied to a prophecy that could change the fate of the wizarding world. It's a great twist because it takes something as simple as a weapon and turns it into a major plot device.