One common theme is friendship. Even though it's a love game, their strong bond as siblings and friends is often explored. Another theme is mystery, as the nature of the love game itself is usually a mystery they have to solve.
In these fanfics, the theme of growth is quite common. Mabel and Dipper start as their regular selves but as they progress through the love game, they grow both emotionally and in their understanding of relationships. Additionally, the theme of adventure is often there. The love game takes them on different adventures, whether it's in a magical realm or just within their hometown. And through these adventures, their relationship as siblings and potential love interests evolves.
In this love game fanfic, it could start with Mabel, Dipper and Bil being transported into a magical world that is all about love. There could be different levels in the game. At the first level, they have to figure out each other's love languages. Mabel might be all about gift - giving, Dipper could value quality time, and Bil might be into acts of service. As they progress, they face challenges that test their relationships and they learn more about themselves and each other in the process.
One possible plot could be that Mabel and Dipper get trapped in a strange game world created by Bill. Bill, being his mischievous self, tries to manipulate the rules of the game to turn Mabel and Dipper against each other. But their strong bond of love for each other as siblings helps them see through Bill's tricks and they start to find ways to turn the tables on him within the game. Maybe they use the power of love, which is something Bill doesn't understand, to break free from the game world and defeat Bill once and for all.
A possible plot could be that an evil entity traps Mabel, Dipper and Bil in a love game. The rules are strange and they have to find true love within the game world to escape. Mabel might start by trying to set up Dipper with a character in the game, but then realizes she has feelings for him herself. Meanwhile, Bil is trying to figure out the true nature of the game and stumbles upon some secrets that could help them all.
One common theme could be the exploration of a different kind of relationship between them, perhaps in an alternate universe where the normal sibling boundaries are a bit more blurred. Another theme might be the idea of a first kiss and the emotions that come with it.
A common theme is growth. Since they are teenagers, it often shows how Mabel and Dipper are growing up, facing new challenges, and becoming more mature.
One main theme is friendship. Despite the new environment of college, Dipper and Mabel's strong friendship endures and is often a central focus. Another theme is growth. They are growing up and learning to deal with more adult - like situations in college.
One common theme is sibling adventures. Since Dipper and Mabel are siblings, a lot of fanfictions explore their relationship further while they go on different adventures in Gravity Falls. Another theme is mystery - solving. It's in line with the overall tone of the show, so fanfic writers often create new mysteries for the twins to solve.
A common theme is friendship. Despite their differences, they learn to be friends. Another theme could be growth. For instance, Pacifica may grow to be more accepting of Dipper and Mabel's wacky ways, and Dipper and Mabel may grow in understanding of Pacifica's background.
Mabel would likely be developed as the heart of the story. Her natural inclination towards love and friendship would be amplified. She might be the one who first reaches out to the other characters in the game, trying to spread love and positive emotions. Dipper, on the other hand, would be more of the strategist. He would use his intelligence to understand the game's rules related to love, like how love can be a power - up or a weakness. Bill would start as the antagonist, trying to disrupt the love in the game. But as the story progresses, he could be slowly influenced by Mabel's love, leading to a change in his character.
One great Dipper and Mabel love fanfiction could be 'The Sibling Bond Transcended'. It explores their relationship in a very heartwarming way, showing how their love for each other as siblings can be so deep and meaningful. It has some really cute moments where they support each other through various adventures.