By focusing on the key moments from the trailer. For instance, if the trailer showed a powerful new weapon, the fanfiction might have characters speculating about its power and how it will be used in the movie. It could also show characters' reactions to the new looks of the heroes, like Thor's new physique, which was quite different in Endgame.
Some of them might be really amused. For example, Thor would probably laugh heartily at any creative interpretations of his character. He might say 'This Midgardian has a wild imagination!'.
In fanfiction, some Avengers might be shocked. For example, Thor could be angry seeing Thanos' looming threat more clearly in the trailer. He might vow to stop him immediately.
In fanfiction, the trailer might inspire the Avengers. Spider - Man's youth and energy could motivate them to step up their game. They might see his unique abilities and think about how to incorporate new strategies into their own fighting styles. Also, it could lead to some of them wanting to meet Spider - Man and offer their help or form an alliance.
Well, they might be really intrigued. Tony Stark would probably start making sarcastic comments about how the symbiote looks. Steve Rogers would be more cautious, thinking about the potential danger it could pose. And Thor might just see it as another strange Midgardian thing.
Some might be amazed. For example, Hawkeye would be impressed by Iron Man's new flying maneuvers. Others like Falcon could be taking mental notes on Iron Man's technology for his own gear. And Tony Stark himself would probably be enjoying the attention and bragging a bit about his new suit's features.
The Avengers' reactions in the fanfiction could vary greatly. Clint Barton might feel a sense of pride seeing his friend Natasha in the Black Widow trailer. He's known her for a long time and would be happy to see her getting her own story spotlighted. Bruce Banner would be a bit more cautious. He might worry about the potential danger Natasha could be facing based on what he sees in the trailer. As for Natasha herself, if she's included in the 'react' part of the fanfiction, she might be a bit embarrassed or self - conscious about all the attention the trailer is getting within the Avengers group.
It could be Iron Man. Since he was always the brains behind a lot of the Avengers' tech and strategies, he might be the one analyzing every detail in the trailer and coming up with plans.
A fanfic could be centered around the younger Avengers like Peter Parker and Shuri. When they watch the Infinity War trailer with the rest of the team, Peter is in awe and can't stop talking about how cool it all looks. Shuri, on the other hand, is analyzing the technology shown in the trailer and starts giving ideas on how to improve their own gear. The older Avengers are both amused and impressed by their enthusiasm, and it brings a sense of hope and unity to the group as they face the impending threat shown in the trailer.
Well, Tony might be intrigued by the new take on Wanda's powers shown in the trailer. He'd probably start making theories about how her reality - bending could be either a great asset or a huge threat. Steve, on the other hand, would be more concerned about Wanda's mental state. He'd remember how she lost Vision and wonder if this new 'WandaVision' world she's in is a way of her coping or something more sinister.