The Soul Reapers would be very confused at first. They are used to Aizen being a certain way, and Naruto's different personality would be a shock. Ichigo might be curious and see Naruto - Aizen as an ally against the common enemies. Orihime could be one of the first to trust Naruto - Aizen because of his kind - hearted nature that she would sense. And the Espadas would probably be on high alert, not knowing what to expect from this new Aizen.
In a 'naruto acts like gin fanfiction', Naruto might be quieter and more observant. Instead of his usual boisterous self, he could be more like Gin, who is often sly and calculating. Maybe he would have a more subdued way of showing his power and a different approach to his enemies.
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.
One possible plot could be that Naruto, after being exposed to some strange chakra or magic, starts to take on Grimmjow's aggressive and fearless attitude. He might become more impulsive in battle, like Grimmjow, and start to challenge his enemies without much thought for strategy at first. His friends would be confused by this sudden change in his behavior.
Well, one possible plot could be that Naruto starts to adopt Pain's view on justice. He might see the world as a place full of suffering that needs a radical change. He could begin to question the traditional ways of the Hidden Leaf Village and try to implement some of Pain's ideas in a more positive way, like finding a new system to ensure peace that doesn't rely solely on the power of the ninja villages.
If Naruto were to act like Yujiro in a fanfiction, it would change his relationships too. He would no longer be the overly - friendly Naruto we know. He might become more aloof and selective with his friendships. His interactions with Sasuke could be different. Instead of constantly trying to bring Sasuke back with kindness, he might use a more forceful and intimidating approach, just like Yujiro would deal with those who oppose him.
Maybe there's a plot where Naruto is influenced by an external force that makes him act like the Joker. He then starts to target the powerful figures in the ninja world, not to defeat them in a traditional sense, but to expose their weaknesses and make them look foolish, just like the Joker does with Batman. And as he does this, his relationships with everyone around him start to crumble.
Sakura would be worried sick. She'd try to figure out what happened to Naruto to make him act this way. She might try to talk to him, use her medical ninjutsu to check if he's under some kind of genjutsu or has some other problem. She'd be very vocal about her concerns and would try to get the other villagers to help bring Naruto back to his normal self.
Maybe Naruto is put in a situation where he feels that Hiruzen's way of handling things is the best option. For instance, if there is a major crisis in the village that requires a more traditional and experienced approach, Naruto might choose to act like Hiruzen. Also, he could be influenced by the stories he has heard about Hiruzen and wants to fill his shoes in a way.
Maybe Naruto admires Kakashi's skills. Kakashi is a highly respected ninja, and Naruto might want to be as good as him. So he starts acting like him to learn from him.