One common theme is friendship. Given their different backgrounds, seeing them form a bond is always interesting. For example, in many fanfictions, they start off not understanding each other but end up being great friends.
I also like 'When Thor Met Quill'. This fanfiction delves into how Thor and Peter Quill first meet and how they initially clash due to their different approaches to life and heroism. But as the story progresses, they learn from each other and form an unlikely friendship. It's a great exploration of their characters.
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.
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.