In this fanfiction, it could be that Bill has a different view of power and hierarchy. He decides to make Dipper his queen as a way to create a new order. Dipper would then have to deal with Bill's minions, who may not be happy with this new arrangement. Also, Dipper might have to face the reactions of his friends and family, like Mabel, who might be very confused or worried about this transformation.
Well, it could be about Bill having some strange and powerful reasons to choose Dipper as his 'queen'. Maybe he sees Dipper's unique qualities like intelligence or a special connection to the supernatural world in the Gravity Falls setting. He might then use his otherworldly powers to transform Dipper into a sort of queen - perhaps giving Dipper powers or a new status that comes with responsibilities and challenges in this fan - created world.
Obviously, the main characters are Bill and Dipper. Bill is a powerful and often malevolent character in the Gravity Falls universe. Dipper is the curious and adventurous young boy. In this fanfiction, their relationship takes a very unique turn with Bill making Dipper his queen.
Another aspect could be that Bill's transformation of Dipper into a girl has some unforeseen consequences. It could be that there is a magical imbalance in the Gravity Falls world as a result. Dipper, in his new form, might discover new powers or abilities that he has to learn to control, all while trying to find a way to reverse the transformation and stop Bill's machinations.
Well, perhaps it's a story where Dipper gets caught up in Bill's gang's evil plans. They might try to manipulate him into doing their bidding, but Dipper uses his wits to outsmart them. The plot could involve Dipper having to navigate through a series of dangerous situations created by Bill's gang, all while trying to protect his friends and the town he loves.
One possible plot could be that Bill Cypher, in human form, befriends Dipper while hiding his true chaotic nature. As they get closer, Dipper starts to notice strange things about his new friend. Eventually, Bill reveals his true self but by then Dipper has grown to understand him a bit and tries to find a way to either change Bill or at least keep his chaos in check.
One aspect that can make it unique is the exploration of an unusual relationship between Bill and Dipper. It can take the dynamic of their interactions in the original work and twist it in new and unexpected ways.
He could start by checking some popular fanfic websites like Archive of Our Own (AO3). It has a vast collection of fanfics from various fandoms. Just use the search function and type in relevant keywords like 'Queen Dipper' and maybe Bill's name if it's part of the fic's title or tags.
One possible plot is that they are recruited into a secret Suicide Squad - esque team to stop an impending apocalypse. Bill could see it as an opportunity for his own gain, while Dipper is more focused on saving the world. Along the way, they encounter strange creatures and difficult moral choices. Their interactions with the other members of the squad, if there are any, would also add to the complexity of the story.
Perhaps Bill, who has changed his ways a bit, tries to court Dipper in his own unique way. He shows Dipper different dimensions and secrets of the universe that only he can access. Dipper is initially hesitant but is drawn to Bill's charm and the mystery he represents. As they get to know each other more, they find that they complement each other well, and getting married seems like the natural next step for them. Their relationship could be a symbol of two different worlds coming together.
One possible plot could be that Mabel and Dipper get trapped in a strange game world created by Bill. Bill, being his mischievous self, tries to manipulate the rules of the game to turn Mabel and Dipper against each other. But their strong bond of love for each other as siblings helps them see through Bill's tricks and they start to find ways to turn the tables on him within the game. Maybe they use the power of love, which is something Bill doesn't understand, to break free from the game world and defeat Bill once and for all.
The contrast between the two characters. Dipper is a curious and somewhat naive kid, while Bill is a chaotic and powerful demon. This creates an interesting dynamic in fanfiction. Writers can explore how their personalities clash or, in some cases, how they might develop an unexpected bond.