A really interesting plot twist could be that they are turned into half - demons by an enemy who wants to use their new forms against the Hidden Leaf Village. However, instead of being used as weapons, Naruto and Hinata manage to use their half - demon powers in unique ways to not only defeat the enemy but also to bridge the gap between humans and demons in the ninja world. They could start a new era of peace and understanding between the two races.
Their relationship would likely become even more intense. As half - demons, they would have to rely on each other more for support in dealing with their new forms and the challenges that come with it. They might also develop a deeper understanding of each other's insecurities.
In this fanfiction, it could be that Naruto is found to be a prince of a foreign land during one of his missions. As a prince, he is expected to marry for political alliances. However, Naruto is torn between his duties as a prince and his feelings for his friends back in Konoha. He has to find a way to balance both worlds and maybe even change the traditional ways of the foreign kingdom.
One possible plot twist could be that Naruto's transformation into the Juubi is not complete. He might retain some of his human emotions and use them to control the immense power. For example, instead of going on a rampage like a mindless beast, he tries to communicate with the other characters using his new form. He could show that the Juubi's power can be harnessed for good, and that it doesn't have to be a symbol of destruction.
One possible plot could be that an ancient and powerful tailed beast is released. Naruto and Hinata, in their quest to protect the village, end up being the only ones able to bond with it. They then have to learn to control the new power within them while facing opposition from other villages who fear their combined strength.
One possible plot could be that Naruto and Hinata are faced with a great threat to the ninja world that is beyond the power of normal shinobi. In their quest to protect everyone, they discover a hidden ancient power that transforms them into gods. As gods, they have to deal with new responsibilities and also face some jealousy from other powerful beings in the ninja world.
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.
Well, a plot twist could be that while Naruto is saving Hinata and Hanabi, he discovers a hidden power within Hinata that she didn't even know she had. Maybe it gets activated during the rescue and she actually helps Naruto in a way he didn't anticipate. And as for Hanabi, she could be the key to finding a secret passage or something that aids in their escape from the dangerous situation.
One possible plot twist could be that one of the twins has a unique and powerful chakra ability that is unknown to the ninja world. This could attract the attention of villains who want to use the child's power for their own evil purposes.
Well, a plot twist could be that Hinata is actually from a future where something bad has happened, and she came back in time to marry Naruto secretly to change the future. Maybe in the original timeline, Naruto dies or the village is destroyed, and their secret marriage is a key to prevent that. Also, there could be a character who is jealous of their relationship and tries to expose their secret marriage, creating all kinds of chaos and drama.
One possible plot twist could be that an old enemy resurfaces just as they are about to get engaged. This enemy might try to disrupt their relationship by spreading false rumors about Naruto, like he has some hidden dark power that could endanger Hinata. Another twist could be that Hinata's family has some ancient clan secret that surfaces and creates a rift between them and Naruto's side. Maybe the secret is related to a power that the Hyuga clan once had and lost, and they fear Naruto might be involved in its re - emergence in a negative way.