They could be shocked. The Avengers characters are so iconic, and their actors brought them to life. Some of the fanfiction might be really far - fetched, like making Hulk into a ballet dancer or something. The actors would be like 'Whoa, that's not what I expected!'
It's possible that the Avengers actors would have mixed feelings. On the positive side, it shows the popularity of the franchise. But on the negative side, some fanfiction might distort the essence of the characters. For instance, if a fanfiction makes Thor a coward, which is completely against his character. His actor would likely be disappointed. But in general, they would probably recognize that fanfiction is a form of fan expression and creativity, and most of it is done out of love for the characters and the movies.
Thor would probably be the most enthusiastic. He'd see it as a great honor that the Midgardians (humans) are so invested in his story and his character. He might even want to join in on the fun and start coming up with his own ideas for fanfiction, thinking it's a great way to further his legend among the people.
Some Avengers might find it entertaining. For example, Thor could chuckle at the creative ideas about his brother. Others like Captain America might be a bit bewildered. He comes from a more straightforward world of stories, and fanfiction can be really out - of - the - box.
Some Avengers might be flattered. For instance, Spider - Man could be happy that fans are so interested in his part in Infinity War that they write fanfiction about it. Meanwhile, Loki, if he were still alive in the context of the reaction, would probably be using the fanfiction to his advantage, maybe trying to manipulate the perception of his actions in Infinity War within the fan - created stories.
Some Avengers would be happy. Like Thor, he would laugh heartily at the fanart that makes him look like a big, friendly giant. In fanfiction, if he's depicted as a noble and powerful being, he'd be all for it. Black Widow might be more skeptical. She'd analyze the fanart and fanfiction carefully to make sure it doesn't give away any secrets or misportray her. She might be okay with the fanart that shows her in a sleek, stealthy way.
Superman could be quite curious. He might read it to see how people view Batman from an outside perspective. Maybe he'll find some new insights into his friend. Batman himself might be a bit embarrassed or annoyed, depending on how the fanfiction is written. If it's full of inaccuracies, he'll likely not be happy.
Black Widow/Natasha Romanoff would be very observant of the fanfiction. She'd look for how fans perceive her character, especially in terms of her relationships with other characters. If she finds any that she thinks are particularly well - written in terms of her character development, she might be secretly impressed. But if there are any that objectify her in a negative way, she'd be a bit annoyed.
Some Avengers might enjoy it. Iron Man, being the tech - savvy and open - minded one, could be interested in seeing how fans imagine his next inventions or his relationships with other heroes. He might even get some inspiration from the creative ideas in the fanfiction. But then there are Avengers like Thor who might be more confused than entertained. He might not understand the modern writing styles and some of the outlandish ideas in the fanfictions.
They might be confused at first. Since the Avengers are from a different universe in terms of the storylines. They could be intrigued by the different power combinations and relationships presented in the fanfiction.
Captain America could have a more determined look. In fanfiction, he would likely see the trailer as a call to action, a chance to rally the team even more. He'd probably think about how to boost the morale of the Avengers and the rest of the world. His reaction would be about inspiring hope in the face of such a seemingly hopeless preview.
The Avengers would likely be overjoyed. Tony might crack a joke about Coulson's 'resurrection' while also being secretly relieved. Steve would probably be very happy to see his friend again. Natasha might be a bit more reserved in her reaction but still glad. Thor would see it as a great sign and welcome Coulson back heartily.