Well, perhaps a plot twist could be that an old enemy resurfaces but this time they target Hinata to get to Naruto. This forces Naruto to not only protect his new wife but also figure out why this enemy is after her specifically. It could also add a layer of drama if Hinata decides to take matters into her own hands and train secretly to be able to defend herself and Naruto better. Or, there could be a political plot twist where their marriage causes unrest in the ninja world because of Hinata's family status and Naruto's position as a hero, leading to them having to fight against those who oppose their union.
A plot twist could be that Sakura and Hinata actually team up against Naruto in some way. Perhaps they realize that Naruto is being pushed into a relationship by the village elders for political reasons. So, they decide to fake a relationship between themselves to make Naruto see that he should be free to choose his own path and not be pressured into a marriage. This would turn the typical love triangle on its head.
One possible plot twist could be that during their training, they accidentally discover a new and powerful jutsu. Maybe it's a jutsu that combines their unique abilities. Naruto's chakra control could be enhanced by Hinata's gentle fist style in an unexpected way, leading to a whole new form of attack or defense.
One interesting plot twist could be that Naruto initially thought he was only in love with Hinata, but during a dangerous mission where Hanabi's life was on the line, he realized he had deeper feelings for her as well. Another could be that there is a prophecy that foretells of a union between the Uzumaki, Hinata, and Hanabi which would bring great power and peace to the ninja world, forcing the characters to consider this unconventional relationship.
One possible plot twist could be that Naruto and Hinata have to quickly learn to control their newfound Jinchuuriki powers. Maybe they are suddenly thrust into a situation where an ancient evil is awakened and only their combined Jinchuuriki abilities can stop it. For example, they might discover that there is a hidden prophecy about two Jinchuuriki working together to save the world.
One possible plot twist could be that Naruto discovers a secret conspiracy against Hinata's family while being her bodyguard. Maybe the real threat is not from an obvious enemy but from within the village's own political circles. He then has to protect Hinata not just from physical harm but also from being used as a pawn in this political game.
In a 'naruto marries sally fanfic', perhaps there's a magical event that links their two worlds. Sally might be a mage from another dimension where she accidentally casts a spell that transports her to Naruto's world. Naruto is instantly intrigued by this strange girl. They start training together, and as they face challenges side by side, they fall in love. The story could end with a grand wedding where Naruto wears traditional Japanese wedding attire and Sally wears something from her own culture.
One plot twist could be that they have to keep their relationship a secret at first due to the wizarding society's conservative views. Another could be that Voldemort uses their relationship as a way to divide the wizarding community. For example, he could spread false rumors about their union being against wizarding laws.
One possible plot twist could be that Blaise Zabini's family secrets come to light during their relationship. Maybe there are dark family alliances or hidden magical artifacts in his family's history that impact their marriage. For example, an old family curse that starts to affect Harry.
Well, a plot twist might be that when Naruto arrives to save Hinata, he finds out that Hinata has already made a deal with the Kumo ninjas for the sake of Konoha. So Naruto has to figure out how to honor Hinata's decision while still protecting her. Also, it could be that the Kumo situation is just a cover - up for a much larger conspiracy involving other villages. And there's a chance that the person Naruto thought was Hinata in danger was actually a clone, and the real Hinata is in an even more dangerous situation elsewhere.
Well, it could be that Naruto made a really stupid mistake that hurt Hinata's feelings deeply. Maybe he accidentally said something really mean about her family or her skills in front of everyone. So in a fit of anger, Hinata slaps him to show her extreme disappointment.