If Naruto acts like Madara in the fanfic, he may begin to view the ninja world in a more cynical way. He could start to question the current system of peace and might try to reshape it according to his own vision, which would lead to battles with those who oppose him. Also, his relationships with characters like Sasuke would take a new turn, perhaps becoming more of a rivalry based on different ideologies.
The fanfiction could explore Naruto being in a situation where he has to act like Madara to deceive the enemy. He gradually gets too deep into the role and starts to actually think and act like Madara on a more permanent basis. His friends notice the changes in his personality, like his increased aggression and his new - found belief in power as the ultimate factor in achieving goals. They then try to bring the old Naruto back.
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.
Yes, in some fanfics Naruto can be depicted as being like Madara. Writers might show Naruto having similar power levels or a certain ruthlessness that is associated with Madara. It could be an interesting take on his character, exploring a darker or more powerful side of Naruto that is not as prominent in the original series.
One common way is in terms of power. In fanfics, Naruto might be given powers similar to Madara's, like his ability to control the Sharingan or some other powerful jutsu. Another way could be in their leadership style. Naruto might be shown leading in a more authoritarian way like Madara did, instead of his usual collaborative style in the original series.
In this kind of fanfic, the author may describe Naruto's resemblance to Madara in terms of their auras. Naruto could have an aura that is as intimidating as Madara's. For example, when he enters a room, people might feel the same kind of pressure as they would when Madara was around. And perhaps Naruto starts to have some facial expressions that are similar to Madara's, like a certain smirk or a serious frown.
He could have been exposed to some dark secrets about the ninja world that made him lose faith in the traditional ways. Madara had his own reasons for his actions, and if Naruto discovers similar things, he might start acting like him. For example, if he finds out that the villages have been using ninjas in unfair or unethical ways for a long time, he could turn to more extreme measures like Madara did.
Another plot point might be Naruto's change in relationships. Since he's acting more like Kakashi, his interactions with Sakura and Sasuke could change. He might be more distant in a Kakashi - like way, which could create tension and new story arcs. Also, the way he deals with the younger generation of ninjas could be different. He could be more like a strict but fair mentor, like Kakashi, and this could lead to some interesting training and growth stories for the new ninjas.
In a Dark Naruto acts like Gaara fanfic, perhaps the story could revolve around a new threat to the village. Dark Naruto, emulating Gaara, decides to take matters into his own hands in a rather ruthless way. He uses his new - found Gaara - like attitude to face the enemy. This could mean using more aggressive and unorthodox combat techniques. Along the way, he might learn some valuable lessons about himself and the importance of his true nature, as opposed to the Gaara - like persona he has adopted.
Maybe he tries to use a jutsu in a completely wrong way. Like he might try to perform the Rasengan but ends up spinning around himself like a top instead. Another example could be that he misinterprets a simple mission instruction and goes off in the wrong direction, thinking he's doing the right thing.