I'm not sure specifically who 'Gregory Green' is in this story without more context. It could be an ordinary person with his own unique experiences, or perhaps a character created for a particular fictional narrative.
Another theme is his passion for music. His garage band is a big part of his character, so a lot of fanfics focus on his musical journey, like writing new songs, dealing with band members, and trying to get gigs. It gives more depth to his love for rock music.
In this type of fanfiction, the character development of Rodrick within the context of the relationship with the OC is important. Maybe the OC brings out a different side of Rodrick, like a more sensitive or responsible side. It could also show how Rodrick's normal antics, such as his band activities or his relationship with his family, are affected by the presence of the OC.
Rowley is a must - include. He and Greg have such a unique friendship. His naivete can contrast well with Greg's more cynical nature. Manny, Greg's little brother, can also be interesting. His cute but sometimes annoying behavior can be a great addition. And let's not forget Fregley. His strange habits can bring a lot of comical moments to the fanfiction.
Well, friendship is a common theme. Fans like to write about Greg making new friends or having issues with his existing ones. His dreams and aspirations also show up a lot. Maybe he wants to be a famous artist or something, and fanfiction can expand on that. And of course, his embarrassing moments are always a hit. These are the things that make the Greg Heffley character so relatable in fanfiction.
One popular 'greg heffley fanfic' might be those that explore his adventures outside of the main Diary of a Wimpy Kid storyline. For example, fanfics where he goes on a mystery-solving adventure in his neighborhood.
Since the 'Diary of a Wimpy Kid' series focuses on Greg's normal, albeit wimpy, life as a kid with his family around, the concept of him becoming an orphan seems quite foreign. Maybe in an alternate universe created by fans, there could be a story where something tragic happens to his family and he becomes an orphan. But in the canonical books, we see his family's influence on his growth, development, and the comical situations he gets into, so it's not a part of the original storyline.
Greg could be a great character to pair with Roderick. Their brotherly relationship has so much potential for interesting stories. They could go on adventures together or get into all kinds of trouble at home or in the neighborhood.
There might be a fanfiction where Roderick tries to teach Greg some cool tricks to be more popular in school. He could use his own experiences, both good and bad, to help Greg out. For example, he might show Greg how to dress in a more 'punk' style to stand out.
Communication is key. They can start by sharing their fears and hopes. For instance, Gregory can talk about his loneliness before coming to this place, and Vanessa can share her own struggles with her job. This mutual understanding will bring them closer.