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.
Another theme is his attempts at romance. Peter is often shown trying to woo Gamora or other characters and failing spectacularly. His clumsy attempts at flirting and showing his feelings end up making him feel embarrassed. In these fanfictions, his big heart and lack of finesse in matters of the heart are highlighted.
Common themes in Peter Quill mpreg fanfiction can be quite diverse. There could be a theme of family, not just in the sense of the new family he is about to have, but also how his existing family (like his mother's legacy or his connection to the Ravagers) plays into this new situation. Additionally, the theme of acceptance, both self - acceptance and acceptance from his friends, is likely to be explored. It could also involve themes of adventure and how this new state of being affects his role in the Guardians' adventures.
One common theme is his origin story. Writers often explore how he ended up as an orphan on Earth and his early days with the Ravagers. For example, many fanfictions might show how he coped with the loss of his mother at a young age.
One common theme is emotional trauma. Peter has been through a lot in his life, like losing his mother and being abducted from Earth. Fanfictions often explore how these past events still haunt him and cause him emotional hurt.
Friendship is also a big theme. The OCs form bonds with Peter Quill and the other Guardians of the Galaxy. They learn to trust and rely on each other in the face of various threats. Additionally, redemption can be a theme. Sometimes the OC has a troubled past and through their interactions with Peter and the gang, they strive to make up for their past mistakes.
Friendship is also a big theme. He forms bonds with humans, and these relationships are often at the core of the story. These friendships help him to better understand Earth and its inhabitants, and at the same time, the humans he befriends are influenced by his unique perspective and abilities.
A sense of discovery is common too. Peter discovers more about the wider universe through Thor, and Thor discovers more about the modern Earth through Peter. And of course, there's the theme of heroism. They both are heroes in their own right, and in the fanfiction, they usually come together to do heroic deeds, whether it's saving a city, a planet, or the whole universe.
One common theme is redemption. Yondu had a rough past as a Ravager, and in many fanfictions, his relationship with Peter helps him find redemption. For example, he might change his ways to protect Peter more wholeheartedly.
Friendship is a very common theme. They are always shown supporting each other through thick and thin. Another one is adventure. Since they are part of the Guardians of the Galaxy, most fanfictions involve them going on wild adventures in space.
One common theme is power amplification. Writers often explore how Peter Quill could become even more powerful, like getting new abilities related to his Celestial heritage.