Another possible theme is the exploration of different cultures within the Adventure Time universe. Lady Rainicorn comes from a different species, and fanfictions can delve into how her culture and Finn's human - like culture interact and influence each other.
One interesting fanfiction could be about Finn and Lady Rainicorn going on a magical quest together. They might encounter strange creatures and have to use their unique skills to overcome obstacles. Finn's bravery and Lady Rainicorn's flying ability would be really useful in such a story.
A possible plot could revolve around the baby's special abilities. Since Lady Rainicorn has her own set of abilities, it's likely the baby will inherit some. In the fanfiction, the characters could be speculating about what those abilities might be. And as the pregnancy progresses, there could be some signs or events that give hints about the baby's future powers. This could lead to a lot of anticipation and excitement among the characters.
Finn would probably be very protective. He might stand guard outside her resting place and keep a lookout for any potential danger. Jake, on the other hand, could use his shape - shifting abilities to make things more comfortable for Lady Rainicorn, like creating soft beds or warm blankets. Princess Bubblegum might use her scientific knowledge to make sure Lady Rainicorn has a healthy diet and proper medical care.
A possible plotline could be Finn getting lost in a magical forest all by himself. He has to find his way out while facing strange creatures and magical obstacles.
Finn is the adventurous and somewhat naive human. His perspective is always interesting as he views the magical world of Ooo through the eyes of an outsider. The lady, whoever she is, usually has her own set of values and goals. If it's Flame Princess, for example, she has her power and her own kingdom to think about. Their conversations in the fanfiction are shaped by these aspects of their characters.
A cool plot could be that Jake and Lady Rainicorn discover a hidden dimension. In this dimension, they find out that there are versions of themselves with different powers. They have to work together to find a way back home while also learning from these alternate versions. Or perhaps a story where they are invited to a royal ball in the Candy Kingdom but end up getting caught in a mystery involving a missing jewel. They then use their unique skills to solve the mystery.
One way is to show them sharing their secrets. Lady Rainicorn could tell Jake about her family in the Rainicorn Kingdom and Jake could share his dreams for the future. They could also have arguments over small things like what to have for dinner, which shows their closeness and comfort in being honest with each other. Then, they make up and their bond becomes stronger. Also, they can protect each other from danger. When Jake is in trouble, Lady Rainicorn uses her horn's powers to save him and vice versa.
He could cry due to unrequited love. Finn has had his fair share of romantic interests in the show. In a fanfic, if the girl he loves doesn't return his feelings, it would be a heart - breaking situation for him. Also, in some fanfics, Finn might be confronted with the harsh reality of the world he lives in. Maybe he discovers something truly dark about the Land of Ooo that shatters his innocent view of it, causing him to break down in tears.
One common theme is self - discovery. Finn often goes through a process of learning more about himself as he experiences love. Another theme is friendship. Since Finn has so many friends in Adventure Time, his relationships with them are often intertwined with his romantic interests.
One interesting theme is Finn's search for identity. Since he's a human in a strange land full of magical beings, he often questions who he is and where he belongs. Another theme could be his relationships, like his brotherly bond with Jake or his complex feelings towards Princess Bubblegum.