He could be a mysterious outsider. Just suddenly appear in the Naruto world and start causing trouble. Maybe he's looking for something in the Naruto universe, like a powerful chakra source or a special jutsu. And the ninja have to figure out who he is and what he wants before he destroys everything.
Bill Cipher could be a part of an ancient prophecy in the Naruto world. The prophecy could say that a being from another dimension will come and either save or destroy the ninja world. And it turns out to be Bill. This would give the Naruto characters a reason to interact with him and try to influence his actions.
In a fanfiction, Bill Cipher could be a hidden power that was sealed away long ago in the Naruto world. When accidentally released, he starts to reshape the world according to his own chaotic vision. Naruto, with his unwavering belief in his friends and the power of the Nine - Tails, would be the one to stand up against Bill and try to return the world to normal.
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.
In a 'Naruto is Bill Cipher' fanfic, the story could start with some sort of interdimensional event. Naruto could be accidentally transported to the Gravity Falls universe where he somehow merges with Bill Cipher. When he returns to his own world, he has Bill Cipher's abilities but also some of his personality traits. This would lead to a lot of confusion among his comrades. The fanfic could then focus on Naruto's internal struggle between his original self and the influence of Bill Cipher. He might have to learn to control these new powers without losing his sense of self and also deal with the reactions of his friends and enemies in the ninja world who are both frightened and intrigued by this new Naruto.
Well, it could be a really interesting creative crossover. Bill Cipher from Gravity Falls is this super powerful and chaotic being. In a Naruto fanfic, he could bring a whole new level of chaos to the Naruto universe. Maybe he'd be a new kind of enemy that the ninja have to face, one that doesn't play by the normal rules of chakra and jutsu. It could also be a chance for some really cool character interactions, like how would Naruto react to Bill's mind - games and illusions.
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.
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.
First, give the character a strong motivation. For example, if the character is a scientist, their motivation could be to study Bill Cipher's otherworldly powers. This will drive their actions and reactions when interacting with him. Also, give them flaws. Maybe they're overly curious which sometimes gets them into trouble when dealing with Bill.
One popular plot line is Dipper's quest to stop Bill from taking over the world. Dipper uses his smarts and the help of his friends to try and foil Bill's plans.
One possible plot could be that Bill takes Mabel to his dimension. There, Mabel has to use her wits and creativity to outsmart Bill. Maybe she befriends some of the strange creatures in Bill's realm who help her in her escape plan.
Ford Pines is another likely character. Given his knowledge of the strange and the supernatural, his encounter with Bill Cipher in a crossover could be full of tension and intellectual battles. He might try to outwit Bill using his scientific knowledge combined with his understanding of the occult.