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.
The unique aspects are many. For starters, their different species make for an interesting dynamic. Peter being human and Rocket being a raccoon - like being. This leads to situations where they misunderstand each other's cultural or species - specific behaviors. Also, in fanfiction, their friendship is often explored in the context of the larger Guardians of the Galaxy group. How they support each other while also having their own roles within the team. Moreover, the fanfictions often play with the idea of their loyalty to one another. Despite their bickering, they always have each other's backs, and that's a great theme to explore.
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.