Some would be curious. Hawkeye might pick up on the unique skills and abilities given to the Team Red characters in the fanfiction and compare them to his own. He could either be inspired or find it a bit far - fetched.
The Avengers' reactions would vary greatly. Black Widow might analyze the character relationships in the Team Red fanfiction from a strategic point of view. She could be impressed by how the fans have explored the potential interactions between different characters. Hulk might not care much at first, but if the fanfiction involves some kind of epic battle scenarios, he might show a bit more interest and grunt in approval or disapproval depending on how well - written he thinks the action is.
In fanfiction, when the Avengers meet Team Red, there could be a lot of chaos. Some members from each side might not trust the other. However, there could also be some really interesting character interactions. Like Iron Man might be impressed by the tech of a Team Red member, while Captain America tries to keep everyone in line. Eventually, they could form an alliance to save the world from a new threat.
In this fanfiction, some Avengers might be in shock. For example, Captain America could be deeply saddened, his belief in justice and protecting the world shaken. He might feel a sense of failure for not being able to prevent the Snap.
It gives fans a chance to explore different 'what - if' scenarios. By seeing the Avengers' reactions, fans can get a deeper understanding of the characters.
Tony Stark might use his quick wit and technology. He could immediately scan Peter to make sure there's no physical danger causing the hyperventilation. Then he'd probably crack a joke to try and calm Peter down, like 'Kid, you breathe any faster and you'll fly away like a human jetpack.'
Some characters would likely be entertained. For instance, Loki might enjoy seeing the various ways fans write him as a more complex anti - hero. He'd probably find the creativity in the fanfiction quite appealing.
Sakura would be heartbroken. She'd likely spend hours upon hours by Naruto's side, trying to jog his memory with stories of their adventures together. She might even get a bit overbearing in her attempts. Sasuke, who has his own complex relationship with Naruto, would be conflicted. He'd see Naruto's forgetfulness as a sign of weakness at first but then realize the importance of their bond. Kakashi, being the experienced leader, would start by observing Naruto carefully to see exactly how much he has forgotten and then start a systematic process of helping him remember.
It can also be frustrating at times. Some fanfictions lack the proper understanding of the movie's lore. They might introduce concepts that don't fit well with the established Marvel Cinematic Universe. But on the positive side, many fanfictions are a labor of love. Authors pour their passion for the movie into their writing, and that can really shine through, especially when they capture the essence of the characters as we know them from the movie.