The Hidden Leaf villagers would be shocked. They see Akatsuki as the enemy, so they would think Naruto and Sasuke have betrayed them. Kakashi would probably be very concerned and try to find out their reasons. Sakura would be heartbroken and might even try to go after them to bring them back. As for the Akatsuki members themselves, some might be suspicious of Naruto and Sasuke's motives at first, while others might see it as an opportunity to use their power for Akatsuki's goals.
The Akatsuki members themselves would have mixed reactions. Some, like Itachi (if he were still alive in the fanfic), might see Naruto as a potential asset but also a threat to their long - term goals. Kisame might be amused by Naruto's presence and see him as a new source of entertainment. Meanwhile, the Leaf Village elders would be outraged. They would see Naruto as a traitor and start devising plans to either bring him back or eliminate him.
The ponies' reactions would vary greatly. Rarity might be fascinated by Optimus Prime's design and might even try to make some sort of fashionable accessory inspired by him. Applejack would be more practical and would wonder what he was doing in Equestria and if he had any good intentions. Optimus would see the ponies as a new and interesting species. He would be patient with their initial wariness of him and would try to show them that he means no harm. He might also be a bit taken aback by their colorful and somewhat chaotic world compared to the more structured world of the Autobots.
The characters in One Piece and Sonic are very different. Sonic is a heroic hedgehog from a modern - ish world, while the One Piece characters are pirates, marines, and various other fantastical beings. When they meet, there could be a lot of misunderstandings at first. For example, Sonic might not understand the concept of the Pirate King right away. And the One Piece characters might be freaked out by Sonic's non - human appearance. But as they get to know each other, they would find common ground. Sonic could help the Straw Hat Pirates in their adventures with his speed, and the pirates could introduce Sonic to the wonders of their world, like the different islands and the secrets of the Grand Line.
The other members of the Akatsuki would have mixed reactions. Some, like Itachi (if he were still alive in this fanfiction), might see Naruto as a glimmer of hope for the group to change its ways. He could work with Naruto to steer the Akatsuki towards a more positive path. But others, who are more set in their old ways, might resist Naruto's presence and try to undermine him. The villagers of the Leaf would be shocked and confused. They've always seen the Akatsuki as the enemy, and Naruto joining them would be a huge paradigm shift for them.
Well, there are also those characters who might be a bit shocked or confused. Take Levi from Attack on Titan. He's a very serious and no - nonsense type of character. If he were to read some of the fanfic about him that deviated too much from his own self - perception, like being made into a comical or overly emotional character in the fanfic, he might be taken aback at first. But then he might also realize it's just a different interpretation and not take it too seriously.
The heroes in MHA would likely be initially shocked and a bit intimidated by the Doom Slayer. His brutal and direct approach to combat is very different from their more heroic and often rule - bound ways. Bakugo might be one of the first to be both intrigued and challenged by the Slayer's presence, as he respects power. The villains, on the other hand, would probably see the Doom Slayer as a major threat and might try to avoid direct confrontation at first.
Batman would find the world of Death Note very disturbing. The concept of the Death Note and the power it holds over life and death is something that goes against his sense of justice. He would be on a mission to stop anyone from misusing it. On the other hand, Light from Death Note might see Batman as a threat to his grand plan. Batman's reputation for thwarting criminals would make Light view him as someone who could expose his secret and put an end to his 'new world' vision.
Well, it depends on the nature of the fanfiction. Some anime characters might be amused if the fanfiction is light - hearted and full of comical situations. For example, if it's a fanfic about a cooking competition among the characters from a shonen anime, they might react with good - natured laughter and competitiveness. They could also be confused if the fanfiction takes their story in a very strange or unexpected direction, like suddenly making a character from a historical anime end up in a space adventure.
The Naruto characters would be shocked by the brutality and hopelessness of the Walking Dead world. Naruto, with his positive attitude, might try to inspire the survivors to keep fighting. The Walking Dead characters, on the other hand, would be in awe of the Naruto characters' powers. Rick might see Naruto as a potential leader with his ability to bring people together. Daryl would probably be interested in learning how to use a weapon like a kunai or shuriken.