I haven't actually read this specific 'piccolo and gohan lemon fanfiction', but generally, in fanfictions involving these two characters, it might explore their relationship in unique ways. It could be about their friendship evolving, or perhaps their teamwork in new and unimagined scenarios within the Dragon Ball universe.
I'm not that familiar with the specific 'Brian Piccolo Thanksgiving Day story'. Maybe it could be about how he spent the Thanksgiving Day with his family, sharing a big meal and giving thanks. It might also involve some special traditions he had on that day.
I'm not entirely sure which specific 'piccolo x gohan lemon fanfic' you're referring to. But generally, if it's a fan - made story, it might explore unique relationships and character developments. It could potentially show how Piccolo and Gohan interact in a different way than in the original series, perhaps delving into their friendship or a more complex emotional bond.
One idea could be to focus on Bardock's perspective before his fateful battle. Maybe explore his thoughts about his son Goku being sent to Earth and his own impending doom. He could have some premonitions about Goku's future role in saving the universe.
Training is also a popular theme. Since Piccolo was Gohan's mentor for a while, fanfics often show them training together and Gohan achieving new levels of power with Piccolo's guidance. There are also themes about family, as Piccolo becomes like an honorary family member to Gohan. He protects Gohan and is there for him in a way that is similar to a family member, and this aspect is often explored in fanfics.
One possible theme could be growth. Gohan has always been on a journey of growing up and Piccolo has been part of that journey. In the fanfiction, this growth could be explored in a more personal and emotional way, perhaps related to their relationship.