Sakura would be shocked and heartbroken. She has always seen Naruto as a positive force. She would try to understand what led Naruto to this change and might attempt to use her medical ninjutsu to see if there's some sort of mental or physical ailment causing it. And she would definitely be among the first to oppose Naruto if he starts doing harmful things.
Shikamaru, being the intelligent one, would try to analyze the situation first. He would understand that Naruto isn't himself and would start to plan a way to reverse whatever happened. He might try to gather the other ninja to form a strategy. Kakashi would be deeply disappointed. He has trained Naruto and seen his growth, and this change would be like all his efforts going to waste. He would try to talk to Naruto, using their past experiences to try and break through to the real Naruto inside.
Sasuke would be furious. He has a deep connection with Naruto, and he would immediately start looking for Naruto. He might go on a solo mission to find and rescue Naruto, using all his skills and powers. Sasuke would not rest until he got Naruto back safe.
If Naruto has leukemia in a fanfiction, his friends' reactions would be complex. For example, Kiba might be angry at the situation, feeling that it's unfair to Naruto. He would try to find ways to make Naruto feel better, like taking him on walks with Akamaru. Rock Lee would be inspired by Naruto's fight against the disease, seeing it as another form of training. Tenten would help by getting any items or herbs that might be useful for Naruto's treatment. And all of them together would form a strong support network for Naruto, constantly reminding him that he's not alone in this fight.
In such a fanfic, Naruto's origin story would be completely rewritten. He could have access to Uchiha clan secrets and powers much earlier. This would change his relationships with Sasuke and other Uchiha members. Also, the story could explore how Madara's influence, even from the past, affects Naruto's decisions and his path as a ninja. There would be new political implications in the ninja world as well, considering Naruto's new family connection.
Well, it would be really different from the original story. Maybe Naruto would inherit some of Madara's powers and have a darker side to him. He might also face a lot of inner conflict, since Madara was a complex and often villainous character.
His relationship with Sasuke would be really interesting. Since Sasuke has his own complex connection to the Uchiha clan and Madara, Naruto becoming a Madara fan could either bring them closer together in understanding the Uchiha's past or drive them further apart if Sasuke doesn't agree with Naruto's new - found admiration. Maybe Naruto would start to push Sasuke towards a more 'Madara - like' view of power and revenge.
There would be a power shift. Since Madara was extremely powerful, Naruto - as - Madara would be an almost unstoppable force. Other villages would have to either ally with Konoha (which would be a difficult decision considering the situation) or try to find a way to counter this new threat. This could lead to a new arms race among the ninja villages.
It would have a huge negative impact. Naruto's friends have come to rely on his emotional support and his ability to inspire them. Without his emotions, he might not show the same enthusiasm in helping them. His friendship with Kakashi would also change as Kakashi has guided Naruto in part based on his emotions and his will to protect his friends. Naruto might seem distant and unresponsive, causing his friends to feel rejected.
Well, it could be due to some dark influence. Maybe Orochimaru gets his hands on some ancient jutsu that can corrupt Naruto's mind and make him adopt Madara's ruthless ways. For example, if Orochimaru manages to tap into the same power source that once drove Madara to seek power at all costs.
If Naruto were to become like Madara in fanfiction, it would be a huge shift. Naruto has always been about hope and friendship. But like Madara, he could start to see the ninja world as something that needs to be reshaped by his own hands, regardless of the cost. He may abandon his village's values and try to impose his own version of order. This would lead to conflicts with his former allies. For example, Sasuke might be one of the first to oppose him as Sasuke has his own ideas about the ninja world and would not easily accept Naruto's new Madara - like ways.