One acceptable theme is their relationship in terms of power balance. How Piccolo's different forms compare to Cell's various stages of evolution. Also, the psychological aspect of their encounters, like how they perceive each other as opponents, can be a great theme for fanfiction.
Writing such fanfiction that involves inappropriate and non - canon sexual content is not appropriate. Dragon Ball series is mainly focused on action, adventure, and the development of characters and their powers, not this kind of vulgar content.
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.
It depends on the size and location of the hole punches. Small holes in non-critical areas might not have a huge impact, but large or in key parts could lower the grade significantly.
A major theme is identity. Since she is a female version of Piccolo, she has to come to terms with who she is compared to the original Piccolo. Family also plays a role. In some fanfictions, she might be searching for her family or creating a new family of her own. Additionally, the theme of good vs. evil is common, as she battles against various villains.
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.