I think it could be Clint. He has a family and the thought of his family being Snapped away would be devastating. He would be haunted by the idea that he wasn't there to protect them. He might feel a great sense of guilt and loss compared to the others.
In my opinion, it would be Steve Rogers. He has always been the symbol of hope and leadership for the Avengers. Watching the Snap happen under his leadership, even if it was out of his control, would make him feel like he has failed. He would feel a heavy burden to try and fix this, and the guilt would eat at him.
Natasha could also be greatly affected. She is perceptive and if she reads Tony's thoughts, she might use that knowledge to manipulate situations within the team for the better or worse. Maybe she discovers Tony's concerns about a potential threat and uses her skills to help address it.
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.
Probably Iron Man. He's often focused on the task at hand and might be the most annoyed by the interruption.
In this fanfiction, the Avengers would likely be filled with shock. They might stand there in stunned silence, unable to process what just happened. Their expressions would range from horror to disbelief as they watched their friends and allies turn to dust.
Definitely Tony Stark. He's all about his high - tech world and order in his lab. Goose could easily get into his lab and start pawing at his precious inventions. It would drive him crazy as he tries to keep Goose away from his dangerous experiments and valuable tech. Also, Goose might short - circuit some of his equipment with its fur or just by being clumsy around it. And Stark is not really used to dealing with such unruly creatures, so he'd be very affected.
Steve Rogers would probably be one of the most concerned. His sense of duty and protectiveness would make him take the 'get help' situation very seriously. He'd be worried about the safety of those in need.
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.
I think Steve Rogers would be a likely ally. Steve is all about justice and doing what's right, and Castiel has a similar sense of duty in his own way. They could bond over their strong moral compasses.
Iron Man. They both are very intelligent and rely on their technology a great deal. They could share ideas about improving their gadgets and suits.
Iron Man would probably be the most interested. He's always into new technology and Spider - Man's web - shooters would be something he'd love to study and maybe even improve.