They might be really excited, especially if the fanfiction contains interesting new storylines or character developments that they haven't thought of before.
Spider - Man, being the youngest of the group, would be really excited. He would love seeing all the different versions of the Avengers' story. He might even get some inspiration from the fanfiction for his own heroic deeds. He could exclaim 'This is so cool! There are so many ways our story can be told!'
Endgame originated from the Marvel Comics universe. So, in a sense, it can be considered related to comic books. It brings those comic book elements to the big screen in an exciting way.
Definitely. Endgame is deeply rooted in the world of Marvel Comics. The movie builds upon the existing comic storylines and characters, bringing them to life on the big screen in an epic way.