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.