One idea could be that Bill initially tries to force his love on Mabel in a very chaotic and otherworldly way. But as time goes on, Mabel's positive energy starts to have an unexpected impact on Bill, making him question his motives. Maybe he starts to see the value in real, unforced love through her.
In the fanfiction, if Bill wants to have a non - toxic relationship with Mabel, he has to give her the freedom to choose. He could start by showing his good side in a non - threatening way. For example, helping Mabel with her problems without asking for anything in return. Mabel, seeing these positive changes in Bill, might start to see him in a different light. But it's important that this development is slow and based on Mabel's own feelings evolving naturally, not because of any coercion from Bill.
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.
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.
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.