Obviously, Phil Coulson is one of the main characters. He is the one who adopts the Avengers. Then, of course, the Avengers themselves are main characters too. This includes Iron Man (Tony Stark), Captain America (Steve Rogers), Thor, Hulk (Bruce Banner), Black Widow (Natasha Romanoff), and Hawkeye (Clint Barton). Sometimes, other characters like Nick Fury might also play important roles as he is Coulson's superior and has connections to the Avengers.
In some Coulson Adopts Avengers fanfiction, there could be a plot where Coulson helps the Avengers deal with their personal traumas. For example, he might use his father - like wisdom to guide Tony Stark through his self - doubt. Another plot could be Coulson training the younger Avengers like Peter Parker in combat and espionage skills, just like a mentor. And sometimes, there are plots where Coulson has to protect the Avengers from a new, powerful threat that the world doesn't even know about yet.
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.
Well, the Avengers would be shocked at first. But then, they'd probably be really happy. Thor might give Coulson a big, friendly slap on the back. And Black Widow would be curious about how he came back. Maybe Coulson's return would make the Avengers more determined to fight their enemies, since they got their friend back. It could also change the dynamics within the team, like more trust and better cooperation.
I would recommend 'When Agent Carter Met Thor'. It's a unique take on the crossover concept. It explores how Peggy would interact with Thor, a very different kind of character from her own world. It has some great dialogue and action scenes that make it an enjoyable read.
Some relationships would become stronger. For instance, Coulson had a good rapport with Captain America. Their friendship would be rekindled and might even grow deeper. With Coulson back, the Avengers would be more united as a whole. They would have more faith in the idea that no one is truly lost. And Coulson could act as a mediator in any potential disputes among the team members, like between Iron Man and Thor.
It would strengthen the team's bond. Coulson was always a sort of glue that held them together in a way. His return would bring back a sense of familiarity and comfort. They would trust each other more knowing that someone they thought was lost could come back.