Their relationships with their friends would become extremely strained. Their former comrades would be shocked and saddened by their turn to evil. Naruto and Hinata would no longer be seen as the heroes they once were, and their friends would likely try to bring them back to the light, but it would be a difficult and dangerous task.
One possibility could be that they turn evil due to a powerful enemy using some dark magic or jutsu on them. This could make them lose their sense of self and start causing chaos in the Hidden Leaf. For example, they might start attacking their former friends and allies, believing they are now enemies.
Well, in terms of his relationship with Sasuke, it could go either way. Sasuke might see Naruto's transformation into the Kyubi as a challenge to his own power and strive to become stronger. Or, he could be intrigued by this new form of Naruto and consider teaming up with him again. Regarding his relationship with the villagers, those who once feared him for having the Kyubi inside him might now fear him even more in his Kyubi form. However, some of the younger generation might look up to him as a symbol of strength. And his relationship with Hinata would also be affected. Hinata might be more determined to protect Naruto, but also feel a bit inadequate compared to his new - found power.
If Naruto transforms into Hinata, his relationship with Sasuke might change. Sasuke might see a different side of Naruto, a more timid and gentle side like Hinata's. And when he's Neji, his relationship with the Hyuga clan could be different. He might be more involved in their internal affairs as Neji was. Also, his relationship with Shikamaru could be affected as he takes on Neji's more serious and duty - bound attitude.
Well, with Sasuke, their relationship would become more of a protective - protectee one. Sasuke, who usually has a rivalry with Naruto, would now feel responsible for protecting this baby Naruto. With Sakura, she would be more motherly towards him. She might be the one constantly fussing over him, making sure he's clean and fed.
His relationship with Sasuke could become more complex. Sasuke might be both envious of Naruto's new power and also worried about Naruto losing his humanity in the dragon form. Sakura could be initially scared but then determined to help Naruto control his form, which would strengthen their friendship. His relationship with Kakashi would change as Kakashi would now have to mentor Naruto in a different way, guiding him to use his dragon powers for good and not let them consume him.
Naruto and Sasuke's relationship would definitely take a new turn. Naruto, who has always been chasing after Sasuke, might now feel a different kind of protectiveness. Their bond could become more complex, with Naruto having to re - evaluate his feelings. For other characters, like Sakura, she might be initially shocked but then develop a sisterly bond with Sasuke. Kakashi, as their mentor, would have to guide them through this strange new situation and ensure that Sasuke is still able to contribute to the village in his new form.
His relationship with MJ would be completely shattered. Instead of being the loving boyfriend, he could become possessive or even abusive. His friends at school would be terrified of him. Ned, who was once his closest friend, would probably try to avoid him or even try to find a way to stop him. And his relationship with the Avengers would turn into enmity. They would see him as a threat and would try to contain or stop him.
His relationship with Sasuke would be very different. Sasuke might be more protective of Naruto as a little girl. He may not see her as the same rival he once did and instead feel a need to safeguard her.
In this fanfic, their relationship will change in many ways. Hinata moving in with Naruto means they'll be sharing a living space. This will lead to them sharing their hopes, fears, and secrets. Naruto will be able to comfort Hinata during her tough times, and Hinata will be there for Naruto when he has nightmares about his past. They'll also start to develop new traditions together, like having a special dinner every Friday night or going for a walk in the forest on Sundays.
Naruto might become more responsible. He's used to being a bit of a wild card, but with a marriage to Hinata, he realizes he has someone depending on him. So he starts to think more about the future and how his actions affect not just him, but Hinata as well.