Iron Man is a very popular choice. His sacrifice in the movie was so impactful that many fans like to explore alternate endings where he survives or different scenarios leading up to his death. They might write about his relationship with Pepper Potts in more detail or his interactions with the other Avengers in a different light.
You could also write a fan fiction where the Avengers use time travel in a different way. Instead of going back to collect the Stones, they go back to stop Thanos from ever getting the idea of the Snap. This would involve changing key events in Thanos' past, like preventing him from meeting Gamora or finding out about the Infinity Stones.
It could be Captain America. He has a strong sense of justice and kindness. So, if he found a dog, he would surely take it under his wing and protect it. Maybe he'd even train the dog to be a mascot for the Avengers.
In some fanfictions, the 7th Avenger could be a time - traveler from the future. This person has come back to prevent a major disaster that the Avengers couldn't handle without their help. They know all about the future events and use this knowledge to guide the team, but also have to be careful not to change the timeline too much.
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.