One main theme is the clash of different worlds. 'Worm' has a more urban, superpower - filled setting, while 'Gravity Falls' is a small, mystery - ridden town. When they cross over, it's about how these two very different environments and the characters within them interact.
Another one is 'Unlikely Allies'. It delves deep into how the characters from 'Worm' and 'Gravity Falls' would form alliances. The story is full of action and adventure as they face common threats that are a result of the crossover between the two worlds. It's really exciting to read how the powers and the mysteries of both universes come together.
Definitely Dipper and Mabel from 'Gravity Falls' would be main characters. And from 'The Replacements', Todd and Riley could be the main ones. They would be the ones driving the plot forward as they interact with the new and strange world or characters they encounter.
One common theme is the exploration of the 'opposites attract' concept. Bill is this powerful, otherworldly and often malevolent being, while Mabel is a sweet, innocent and positive girl. Their interactions based on these contrasting natures are often explored. Another theme is the idea of the unexpected. Their relationship is not something that one would expect in the normal Gravity Falls storyline, so fics often play with this element of surprise.
The most common theme in 'gravity falls bill x dipper fanfic' is the relationship between the two characters. It can range from a manipulative one where Bill is using Dipper to a more complex relationship where they might have some mutual understanding. Another theme is the mystery surrounding Bill and how Dipper tries to figure it out. Also, the idea of transformation is present in some fics, where Dipper may be changed in some way due to his encounters with Bill, either physically or mentally.
Mystery is also a big theme. Both Naruto and Gravity Falls have elements of mystery in their own right. In the crossovers, these mysteries blend. For instance, the mystery of the Akatsuki's goals in Naruto might get intertwined with the mystery of the Gravity Falls anomalies. And of course, there's the theme of self - discovery. The characters, whether from Naruto or Gravity Falls, learn more about themselves as they interact with the new and different characters from the other universe.
Another common theme is friendship. The main characters in both shows are young and form strong bonds with their friends. In crossover fanfiction, you'll often see the characters from Hilda befriending the characters from Gravity Falls. They might rely on each other's unique skills and knowledge to get through various challenges, like when Hilda uses her wilderness skills and Dipper uses his mystery - solving abilities to face a common threat.
A possible plot is that the 'Gravity Falls' characters accidentally open a portal to the world of 'The Replacements'. The Replacements' characters then have to adjust to Gravity Falls' weirdness. There could be a conflict where the Gravity Falls' villains try to take advantage of the new arrivals, but they all end up working together to save both their worlds.
Character interactions are key. You want to see how the characters from different shows blend together. For example, if it's a crossover with Scooby - Doo, seeing how Shaggy and Dipper would react to the mysteries in Gravity Falls. Also, a good plot that makes sense for both universes. It can't just be a random mash - up.
One good theme could be friendship. Wendy is a really cool character, and writing about her and the reader becoming friends and having fun in Gravity Falls would be great. Another theme could be mystery - solving. They could work together to figure out the secrets of the town.
One of the main themes is mystery. The story in the Gravity Falls Graphic Novel is filled with strange happenings and secrets waiting to be uncovered. Another theme is family. The characters often rely on their family bonds in the face of the weirdness in Gravity Falls.