Joss Whedon, the director behind 2012's The Avengers and its 2015 sequel, Avengers: Age of Ultron, provided a crucial piece of feedback for Guardians of the Galaxy that changed the trajectory of Peter Quill/Star-Lord's character.
In the book, The Story of Marvel Studios: The Making of the Marvel Cinematic Universe, producer Jeremy Latcham explained how advice from Whedon changed an early version of the character Peter Quill, who would first appear in the Marvel Cinematic Universe in the first Guardians of the Galaxy film, which came out in 2014. In an early version of the script, Peter Quill's father was J'son, an emperor, which would make Peter a prince by blood. However, Whedon advised against this storyline.
"Joss sent me a memo in all caps, 'IN NO UNCERTAIN TERMS: YOU DO NOT HAVE A MOVIE,'" Latcham said, who was the executive producer on both Guardians and Age of Ultron at the time. "I didn't want to be developing a movie that was gonna bum him out. It was super heartbreaking because I obviously love and trust Joss."
Whedon's note explained that Peter Quill, portrayed by actor Chris Pratt, couldn't be royalty if Marvel wanted the audience to relate to him as a character; he had to be a normal guy. This note changed Peter's storyline entirely. Instead, as it was later revealed in Guardians of the Galaxy Vol. 2, instead of royalty and his father being an emperor, Quill's father was Kurt Russell's Ego, making Peter part Celestial instead.
Writer/director James Gunn has helmed both volumes of the MCU's Guardians of the Galaxy films and is currently in the process of storyboarding for the third installment, Guardians of the Galaxy Vol. 3. While it has not yet started filming, the film is set to release in May 2023.
The Story of Marvel Studios: The Making of the Marvel Cinematic Universe is comprised of over 500 pages across two volumes of interviews with executives, cast and crew compiled by authors Tara Bennett and Paul Terry, from all corners of what has become the highest-grossing movie franchise of all time. Along with the change to Peter's character, the new book reveals that Mark Ruffalo was initially cast as Bruce Banner/Hulk before Edward Norton beat him out. Of course, Ruffalo would later replace Norton with his appearance in Whedon's The Avengers. In that vein, the book also revealed that Zooey Deschanel almost played the Wasp in the 2012 film, who would have played a major role as part of the superhero team.
Source: The Story of Marvel Studios: The Making of the Marvel Cinematic Universe