One idea could be that Mabel discovers a way to communicate with Bill on a different astral plane. They start off as enemies but as they talk more, they begin to understand each other's motives. For example, Mabel might find out that Bill was once a good entity but was corrupted by some dark force. Another plot could be that Bill, in a weakened state, is found by Mabel and she decides to help him reform against the wishes of her family. Or perhaps there is a prophecy that Mabel and Bill together can save the Gravity Falls universe from an even greater threat.
At first, Mabel could be intrigued by Bill's power. Bill, on the other hand, might see Mabel as a unique being. As they interact more, Mabel could start to empathize with Bill's loneliness. Maybe Bill was always an outcast in his own realm, and Mabel, being the empathetic person she is, tries to befriend him. This could lead to a friendship that is constantly tested by the actions of Bill's past and the expectations of those around Mabel.
One common theme is friendship. Since they are from different backgrounds in the original story, exploring how they become friends is a popular theme. For example, they might learn to understand and accept each other's differences.
A fanfiction might involve a love triangle between Human Bill Cipher, Mabel, and Dipper. Human Bill Cipher, with his manipulative ways, tries to win Mabel's love. Mabel is flattered by his attention but is also conflicted because of her strong bond with Dipper. The story could explore themes of jealousy, loyalty, and the power of true friendship as Dipper tries to protect Mabel from Bill's schemes.
A 'bill cipher x mabel fanfiction' could explore a really strange and interesting relationship. Bill Cipher is this super - chaotic and powerful entity, while Mabel is full of charm and innocence. In fanfiction, it might show Bill trying to manipulate Mabel in his devious plans, or perhaps Mabel finding a way to outwit Bill in some unexpected way. It could also delve into how their different natures interact, like Bill's lack of morals against Mabel's strong sense of right and wrong.
One possible fanfiction could be where Bill Cipher tries to deceive Mabel in a different way than in the show. Maybe he appears to her in a dream, offering her the world's greatest collection of sweaters in exchange for a small favor that's secretly a big deal. But Mabel, being clever, starts to suspect his motives and tries to outwit him using her unique charm and creativity.
Well, in this type of fanfiction, there are numerous possible plot developments. For example, Mabel might discover some hidden power within herself while being kidnapped by Bill. This power could be the key to her escape or to ultimately defeating Bill. Or, Bill could be using Mabel to access a different dimension or unlock some ancient mystery in Gravity Falls. And the Pines family, along with their allies, would then have to race against time to save Mabel and prevent whatever Bill's evil plan is. There could also be a lot of psychological warfare between Bill and Mabel, with Bill trying to break her spirit and Mabel remaining strong and defiant.
Sure. One popular 'Mabel and Bill fanfiction' is 'The Unlikely Bond'. It explores a unique friendship between Mabel and Bill in a really interesting way. Another one is 'Mabel's Adventure with Bill' which has some exciting plot twists.