In a Sasuke - vampire - Naruto fanfiction, a really interesting plot twist could be that the source of Sasuke's vampire powers is actually related to the Nine - Tailed Fox inside Naruto. Maybe there's some ancient connection between the two. This would force Naruto and Sasuke to work together in a whole new way. They'd have to explore this connection and figure out how to deal with it. Also, it could be that Sasuke's vampire side makes him immune to certain jutsu, which could be a huge advantage in battles but also make him a target for those who want to study this immunity.
A possible plot twist could be that Sasuke's vampire condition was actually a curse placed on him by an old enemy of the Uchiha clan. This would add a whole new layer of mystery and revenge to the story. Another twist could be that Naruto discovers he has some sort of power that can either nullify or enhance Sasuke's vampire abilities. It would put their relationship in a new light as Naruto has to decide what to do with this newfound knowledge.
Another plot twist could be that Naruto's own chakra has an unexpected effect on the vampire. Perhaps it changes the vampire in some way, either making them more powerful or causing some sort of internal struggle within them. This would then have a big impact on their relationship as they both have to figure out how to deal with this new situation.
Well, a great plot twist could be that Percy is turned into a vampire but he was actually part of an ancient prophecy in the vampire world. As he struggles to adapt to his new life, he finds out that there are vampire hunters who are after him not just because he's a vampire, but because they believe he will bring about a great change in the vampire - human balance. Also, his transformation could be different from other vampires. Instead of craving blood right away, he has a strange attraction to something else, like the essence of magic, which is very un - vampire - like and causes confusion among the vampire clans.
It could be that Fem Sasuke is under some sort of mind - control when she kidnaps Naruto. A powerful enemy, like Orochimaru or a new villain, might have used a jutsu to make her do this. As the story progresses, Naruto would then have to find a way to break the mind - control on Fem Sasuke while also dealing with the situation of being kidnapped.
A great plot twist could be that Sai is secretly working for an enemy all along. He has been feeding information about Naruto and Sasuke to this unknown foe. This would completely change the dynamics between the three characters.
A great plot twist could be that Neji, who was thought to be a loyal Leaf Village ninja, is actually a spy for an enemy village. This would completely change the dynamics between him, Naruto and Sasuke. They would have to figure out how to deal with this betrayal while also dealing with other threats to the village.
Perhaps Naruto discovers that Kisame has a family he's been protecting in secret, and this new knowledge makes Naruto see Kisame in a different light. He then tries to help Kisame reunite with his family while also dealing with the Akatsuki.
Perhaps Naruto discovers that Sheele was sent to his world by an enemy as a spy, but she has actually fallen in love with the place and the people, especially Naruto, and she has to prove her loyalty. This would create a lot of drama and tension in the story.
A possible plot twist could be that Tsuyu is secretly related to one of Naruto's enemies. Maybe she discovers this during the course of a mission and has to decide whether to side with her newfound family or continue to be Naruto's ally.
One possible plot twist could be that Naruto was cloned by an enemy to infiltrate the Leaf Village. But as he grows up, he starts to develop his own sense of self and loyalty to the village, causing a conflict within him. Another could be that the real Naruto is actually alive somewhere else, and the clone Naruto has to find and reunite with his original self.
A plot twist could be that their deaths were faked by themselves. They might have discovered a great threat to the village that required them to go undercover. So, they faked their deaths to deceive the enemy. Or, perhaps their deaths lead to the awakening of an ancient evil that was sealed long ago. And it turns out that their deaths were somehow a sacrifice to keep this evil at bay for a while, but now the remaining characters have to find a way to defeat it once and for all. This would completely change the direction of the fanfiction.