Harry would be on guard at first. He has faced many dark threats in his life, and Madara would seem like another one. But as he gets to know Madara better, he might see some similarities in their struggles. Dumbledore would likely be very interested in Madara. He would try to understand Madara's motives and see if he could be an ally or a threat to the wizarding world. Dumbledore would be cautious but also open - minded in dealing with this new and strange character.
The Leaf Village would be in chaos at first. Hinata is an important member of the Hyuga clan, and her kidnapping would not be taken lightly. The Hokage would immediately call for an emergency meeting. They would start by sending out search teams, likely led by some of their best shinobi like Kakashi or Shikamaru, who are known for their intelligence and combat skills.
Sakura would probably be shocked at first, but then she'd get angry. She'd immediately start to struggle and try to free herself. She might call out for help if she thinks there are allies nearby.
The other Konoha ninjas would be quite shocked. The Hokage, whoever it is at that time, would see Naruto's change as a potential threat to the village's stability. They would likely keep a close eye on him and try to figure out what caused this transformation. Meanwhile, the younger generation of ninjas who looked up to Naruto would be confused and disappointed. They expected Naruto to be their role model of a kind - hearted and positive ninja, not someone acting like the feared Madara.
Sasuke might be a bit taken aback at first. He's used to Naruto's normal look, and this change could make Sasuke see Naruto in a new light, perhaps as a more formidable opponent or an even stranger ally. Sakura would probably be shocked too. She might tease Naruto about it at first but then grow to accept it. Kakashi would likely raise an eyebrow and make a comment about how Naruto is always full of surprises with his new look.
In such fanfiction, Sakura might be depicted as more alluring and perhaps more bold in her interactions with Madara. Madara could be shown as having a certain charm that draws Sakura to him.
When Madara travels through time, he might find allies in unexpected places. For instance, some of the minor ninja clans that were oppressed by the larger clans in the past could see Madara as a liberator if he offers them a better future. On the other hand, those who benefit from the current ninja world order, such as the Daimyo or certain powerful shinobi from other villages, might see Madara as a threat and become his enemies.
Often, Naruto might be paired with Madara in some sense. Their opposing views on power and peace could create interesting interactions in the fanfiction.
Sasuke could be a main character as well. Given his own experiences of being an outcast and having a thirst for power at times, he could relate to Madara in some ways. They might form an unexpected alliance or have a rivalry based on their shared experiences of feeling neglected or misunderstood in the ninja world.
Well, in fanfiction, the relationship between Hinata and Madara can be developed in many ways. Some might focus on a romantic relationship, where they overcome their differences. Others could explore a more platonic mentorship - like bond. For instance, Madara teaching Hinata some advanced ninja techniques, and in the process, they develop a deep respect for each other. This variety in relationship exploration is part of what makes Hinata and Madara fanfiction so appealing.