Another plot could be that they both get trapped in a time - loop where they relive the same day over and over again. At first, they fight against each other during each loop, but as they start to remember previous loops, they realize they need to cooperate to break the loop. This could involve them exploring their own pasts and how they came to be who they are, and in the process, they form an unlikely alliance.
Maybe Deer Dipper discovers a hidden power within himself that Hunter Bill is interested in. Hunter Bill tries to manipulate Deer Dipper into using this power for his own evil plans, but Deer Dipper resists and tries to turn the tables on him. Along the way, they encounter various otherworldly creatures and challenges that test their relationship.
Perhaps an ancient evil entity that was once sealed away by Deer Dipper's ancestors. It has now been awakened and sees Deer Dipper and Hunter Bill as threats. It uses dark magic and mind - control to try to turn them against each other so that it can regain its full strength and take over their world.
One plot idea could be that Female Bill Cipher, in a more benevolent form, teams up with Dipper to solve a mystery in Gravity Falls that threatens both the human and supernatural worlds. They start off as reluctant allies but gradually grow to respect each other.
One idea could be that Female Dipper discovers a way to enter Bill's dimension. There, she tries to understand his chaotic nature. Maybe she finds a side of Bill that's not entirely evil and they form an unlikely bond. For example, Bill could be facing some sort of threat in his own dimension that only Dipper, with her unique human perspective, can help with.
One idea could be a story where Baby Dipper discovers a magical creature in the forest and Mabel helps him keep it a secret from the adults. They go on adventures to find food for the creature and learn about its magical abilities.
Another aspect could be that Bill's transformation of Dipper into a girl has some unforeseen consequences. It could be that there is a magical imbalance in the Gravity Falls world as a result. Dipper, in his new form, might discover new powers or abilities that he has to learn to control, all while trying to find a way to reverse the transformation and stop Bill's machinations.
One idea could be that Reverse Dipper, coming from his alternate dimension, initially confuses the regular Dipper. Maybe they start by arguing about who is the'real' Dipper. Then, they could team up to solve a mystery that involves both their dimensions.
One idea could be a story where Hermione, through some magical accident, is transported back in time and has to work with a younger Snape and Bill. Maybe they are on a mission to save Hogwarts from a dark wizard before Voldemort's rise. Hermione's modern knowledge of magic combined with Snape's potions expertise and Bill's adventurous spirit could lead to an exciting adventure.
One idea could be that they overcome the initial shock of their relationship from the wizarding community. Since Bill is older and more experienced, he could help Harry deal with his remaining PTSD from the battles. Another could be a story where they discover a new magical bond between them that has implications for the whole wizarding world.
One idea could be Bill Cipher getting involved in a lemon - themed mystery. Maybe there are disappearing lemon groves in Gravity Falls and Bill is suspected, but it turns out to be a mislead and there's a strange lemon - loving creature causing the chaos. Another could be a love story where a character has a lemon - scented charm that attracts Bill in an unexpected way, leading to a wild adventure filled with Bill's usual mind - bending antics.
One plot idea could be that Hermione and Bill team up for a dangerous magical research project. They might have to travel to far - off and dangerous places, facing ancient magical creatures and dark magic along the way. Their different areas of expertise - Hermione's vast book knowledge and Bill's hands - on experience with dangerous magical situations - would make for an exciting combination.