Perhaps Bill wants to use Will as a bargaining chip. He gets Will and then tries to negotiate with some powerful entity in the Gravity Falls universe. In this case, the story could be about the power struggle between Bill, the entity he's negotiating with, and the attempts of other characters to rescue Will.
A more psychological plot could be that Bill Cipher is actually lonely in his chaos and he gets Will Cipher because he believes Will is the only one who can understand him. So, it becomes a story about an unlikely friendship or a twisted relationship between the two, with Bill trying to make Will see things from his perspective while Will resists.
One possible plot could be that Bill Cipher needs Will Cipher to complete a dark ritual. So, he devises a complex plan to capture Will. He might use his mind - bending powers to trick other characters into leading Will into a trap.
One plot could be that the human Bill Cipher gets amnesia and has to figure out who he is while living in a normal human family. He starts to discover his powers slowly and has to hide them from his new family members.
A common plot is Bill tricking Dipper into making a deal. This usually leads to Dipper having to find a way out of the deal without sacrificing too much. It's a classic 'devil's bargain' type of plot that adds a lot of suspense.
I'm not entirely sure specifically what that fanfiction is about. It could potentially be about Bill Cipher achieving or obtaining Will Cipher in some way, perhaps in a strange or otherworldly scenario within the context of the Gravity Falls universe. It might involve some sort of power play or a very unique plot device that only fans who have read it would know for sure.
Perhaps Bill Cipher enters Will's dreams and slowly manipulates his mind. As he 'gets' Will, he starts to change Will's perception of reality. Will, being a more innocent or naive character, might not realize what's happening at first. This could lead to a story where Will has to fight against Bill's control over his mind and soul, creating a tense and psychological plot.
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.
One great Bill Cipher fanfiction is 'The Cipher's Game'. It has an interesting plot where Bill interacts with the main character in a dream - like world filled with strange and mind - bending challenges. The author does a wonderful job of capturing Bill's chaotic and mischievous nature.
One aspect of Bill Cipher and Dipper fanfiction is the exploration of their complex relationship. Bill is often depicted as a menacing and powerful entity, while Dipper is curious yet cautious. In many fanfictions, Dipper might be lured into Bill's schemes, creating a lot of tension and drama. It's a great way to expand on the personalities and potential interactions that were only hinted at in the original show.
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 possible theme could be power imbalance. Since Bill Cipher is often depicted as a very powerful entity, an abused fanfic might focus on how this power is misused against others.
The main difference is the form. In the original, Bill Cipher is his non - human, otherworldly self. But in fanfic, as a human, his physical limitations and advantages are different. He might not have the same instant, all - powerful abilities.