There's a fanfic where child Peter Quill discovers an ancient artifact on Earth. In this story, he starts to show his natural curiosity and his penchant for adventure even at a young age. It's really interesting as it also weaves in some of the alien cultures that he would later encounter in a very subtle way.
One common theme is his sense of loss. Since he lost his mother at a young age in most fanfics, it becomes a central part of his character development. Another theme is his dreams of something bigger. Even as a child, he's often depicted as having a longing for a world beyond his small town on Earth.
The child in fanfiction could potentially have some sort of special power. Maybe it could inherit Peter's ability to hold an Infinity Stone (if we consider that as a latent power) and Gamora's enhanced physical abilities. It would be raised in a universe full of danger and wonder. It could befriend the other Guardians and learn from them. This child might also struggle with the legacy of its parents, having to live up to their great reputations while also carving out its own path in the galaxy.
He might meet his old friends from before he left Earth. People he used to hang out with, like schoolmates or neighbors. They could be shocked to see him after all this time and how much he has changed.
Another theme is friendship or romance. Many fanfics explore the relationship between the female character and Peter Quill. It could start as a friendship based on mutual respect for each other's skills, and then develop into something more romantic as they share more experiences.
It could have a huge impact. If it inherits the best qualities of both parents, it would be a force for good. It might be able to unite different species in the galaxy, just as the Guardians did. The child could bring a new sense of hope and inspiration.
One possible plot could be that they discover Gamora is pregnant while on a mission. They then have to quickly find a safe place to hide and prepare for the baby. Along the way, they encounter old enemies who now see Gamora's unborn child as a valuable asset.
Peter Quill's origin story begins on Earth. He had a difficult childhood as his mother passed away when he was young. After being abducted by Ravagers, he was thrown into a world full of strange and diverse alien races. This experience shaped him into the adventurous and somewhat roguish Star - Lord we know. He learned to survive in the harsh and unforgiving space environment, using his wits and the few Earth - based items he had, like his Walkman, which also held sentimental value as it was related to his mother.
Peter Quill and Mantis aren't siblings in the comic world. There's no established connection of that kind. They have their own unique roles and arcs that don't involve being related by blood.