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.
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 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.