Yes, there are some. You can start by looking on popular fanfiction websites like Archive of Our Own. Just search for 'Mabel Pines x Bill Cipher' and you'll find a variety of stories with different themes and plotlines.
Often, there are themes of friendship and its power. Despite their differences, the fanfictions may explore how Mabel's friendship could influence Bill. Also, the theme of the unexpected can be common, as their pairing is quite an odd one, so the stories play on how unexpected their interactions and relationships can be.
Mabel would likely struggle with telling her family. Dipper might be shocked and not know how to react. Her parents could be disappointed or worried. She would also face challenges related to her own future plans. Maybe she had dreams of traveling or exploring more of Gravity Falls, but now she has to put those on hold for the baby. And there's always the fear of the unknown, like what if there are complications during the pregnancy or if the baby has some sort of special condition related to the strange nature of Gravity Falls.
In a fanfiction plot, Mabel being pregnant could lead to a story where she has visions related to the baby's future. The visions could be connected to the secrets of Gravity Falls. She might go on a quest to find out more about these visions and how they relate to her unborn child. Along the way, she could face challenges from the town's villains who either want to use the baby's power for their own gain or are afraid of the power the baby might possess. This would create a lot of drama and mystery in the story.
I'm not sure specifically as there could be many novels with 'pines' in the title. It might be about a story set among pine trees, perhaps a mystery or a love story in a pine - forested area.
In 'in the pines twilight fanfiction', the pines might symbolize a place of seclusion or mystery. The twilight aspect adds an element of ambiguity and transition. It could be that the story within the fanfiction uses this setting to explore themes like isolation, discovery, or a change in the characters' journey. For example, if it's a fantasy - based fanfiction, magical creatures might emerge in the pines at twilight.