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.
They're popular because it's an unexpected pairing. Mabel is a very positive and kind - hearted character, while Bill is a chaotic and often malevolent force. The contrast between them makes for interesting stories. Fans like to explore how Mabel's goodness could potentially influence Bill or how she would deal with his dangerous nature.
Well, there's a fanfic called 'Mabel and Bill: An Unusual Alliance'. In this one, Mabel finds herself in a situation where she has to team up with Bill to save Gravity Falls from a new, greater threat. It has a really interesting plot with Mabel constantly trying to keep Bill in line while they work together. Then there's 'Bill's Interest in Mabel', which delves into why Bill seems so fascinated by Mabel and how she reacts to his attention.
One popular 'Gravity Falls Bill x Mabel fanfic' is 'The Strange Bond'. It delves into the unique relationship between Bill and Mabel in a really interesting way. It shows how their interactions can be both mysterious and somewhat endearing. Another good one is 'Bill's Fascination with Mabel'. This fic focuses on Bill's rather unusual interest in Mabel and how it unfolds throughout the story.
One possible fanfiction could be where Bill Cipher tries to deceive Mabel in a different way than in the show. Maybe he appears to her in a dream, offering her the world's greatest collection of sweaters in exchange for a small favor that's secretly a big deal. But Mabel, being clever, starts to suspect his motives and tries to outwit him using her unique charm and creativity.
One way a fanfiction could go is that Bill Cipher is actually trapped in a form that only Mabel can see. He needs her help to break free, but Mabel is hesitant. She doesn't trust him at all. Bill Cipher then tries to convince her by showing her glimpses of things she desires, like the future with all her friends happy and her having amazing adventures. But Mabel has to decide if she can really trust Bill Cipher or if it's all just a trick.
One great 'Gravity Falls Bill x OC Fanfic' is 'Bill's New Plaything'. It has an interesting plot where the OC is a powerful witch who catches Bill's attention. The author does a great job of showing the power struggle between them.
Fanfiction.net is also a good place. There are many Gravity Falls fanfics there. However, the quality can vary more than on AO3. But if you search specifically for Mabel x Pacifica, you're likely to find some interesting reads.
Character interaction is key. If the OC and Bill have a dynamic relationship, whether it's a love - hate or a mutual - benefit kind of thing. And the fanfic should stay true to the 'Gravity Falls' universe in terms of the overall feel and the rules of the world. For instance, Bill's chaotic and manipulative nature should be well - represented. Another aspect is the pacing. If it's too slow, readers might lose interest, but if it's too fast, important details could be missed.
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.