One idea could be that as Bill turns human, he has to deal with human emotions like love and empathy for the first time. He might struggle to understand why people cry or laugh, which could lead to some comical and also poignant moments.
Perhaps Bill, now human, tries to use his old otherworldly knowledge to gain power in the human world. But he quickly realizes that human society has its own complex rules and he has to adapt in unexpected ways. For example, he might try to use his mind - reading skills (if he still has some remnants of his old powers) in a business deal, only to find out that it's not as straightforward as he thought.
One interesting idea could be exploring Bill Cipher's interaction with the human world in a more relatable way. For example, how he might adapt to human emotions like love or friendship, which are completely foreign to his usual chaotic nature.
Dipper Pines could be one of the main characters. He might be the one who first discovers Bill's transformation and tries to figure out if it's a trick or if Bill is truly changing. His inquisitive nature would make him a great foil to Bill's new human self.
A plot could be that Human Bill Cipher is actually an undercover agent sent to protect Mabel from a greater threat in Gravity Falls. He has to hide his true identity while getting closer to Mabel. As they spend time together, Mabel starts to suspect that there's more to him than meets the eye.
One idea could be Bill Cipher getting trapped in a human form and having to navigate the strange world of human emotions and social norms. It would be a comical yet thought - provoking plot as he tries to understand things like friendship and love while still having his devious nature.
One idea could be Bill Cipher getting involved in a lemon - themed mystery. Maybe there are disappearing lemon groves in Gravity Falls and Bill is suspected, but it turns out to be a mislead and there's a strange lemon - loving creature causing the chaos. Another could be a love story where a character has a lemon - scented charm that attracts Bill in an unexpected way, leading to a wild adventure filled with Bill's usual mind - bending antics.
One idea could be that Bill Cipher enters the Ladybug universe and tries to disrupt the balance of good and evil. Ladybug and Cat Noir have to figure out his strange powers and motives. Maybe Bill starts by causing chaos in Paris, like making the Eiffel Tower come alive in a menacing way.
One idea could be that Mabel discovers a way to communicate with Bill Cipher in a dream, but instead of being scared, she tries to understand his motives. Another could be a time - travel plot where Mabel and Bill interact in a different era. Maybe Mabel has to team up with Bill to fix a rift in the space - time continuum that he accidentally created.
A great idea for a Bill Cipher crossover fanfiction would be to cross him over with the world of Over the Garden Wall. Bill could be the ultimate evil in that already spooky world. He might try to prevent Wirt and Greg from finding their way home, or he could be after the Beast's power. Another possibility is a crossover with the Legend of Zelda series. Bill Cipher could be a new and powerful enemy in Hyrule, and Link would have to use his skills and the power of the Triforce to defeat him.
A possible plot point is that other characters in the Gravity Falls world try to use Bill's half - human side against him. They might think that his newfound humanity makes him vulnerable and try to manipulate him or find a way to defeat him more easily.
One plot idea could be that Female Bill Cipher, in a more benevolent form, teams up with Dipper to solve a mystery in Gravity Falls that threatens both the human and supernatural worlds. They start off as reluctant allies but gradually grow to respect each other.