Well, in the 'king piccolo meets his son' fanfiction, it could be that the son has some sort of special ability that King Piccolo wants to use for his own evil purposes. However, the son resists. There could also be a sub - plot where other characters from the original story get involved. For example, some of the good guys might try to protect the son from King Piccolo's clutches. And in the end, the son might either change his father for the better or have to defeat him to save the world.
The relationship could start off very rocky. King Piccolo is used to being in control and his son might be more independent - minded. But over time, they could develop a grudging respect for each other. For instance, if the son shows great strength in combat, King Piccolo might start to see him as an equal.
The length of DB mangas differs. It depends on factors like the story's complexity and the author's vision. Some might run for just a few volumes, while others could have dozens.
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.