In a fanfic, their relationship could be developed through cultural exchanges. Since Jake is from Ooo and Lady Rainicorn is from the Rainicorn Kingdom, they could introduce each other to their respective cultures. Jake could learn about the Rainicorn language and traditions, and Lady Rainicorn could experience the unique things in Ooo like the slime pools. They could also have joint hobbies like creating art together using Jake's stretchy powers and Lady Rainicorn's ability to write in the air with her horn. This would bring them closer and deepen their relationship.
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.
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.
A good way to develop their relationship is by introducing a conflict that they have to resolve together. Maybe there is a misunderstanding between them that leads to a rift. But as they work through it, they realize how much they value each other. They could also have moments of vulnerability, like Finn sharing his fears about not being good enough, and Huntress Wizard sharing her insecurities about her magic abilities. This would make their bond stronger.
Another way is through music. Since Marceline is a talented musician, she could teach Ice King how to play an instrument. This shared activity would be a fun way for them to bond. As they practice together, they would get to know each other better on a more personal level. It could also lead to them creating their own unique music that reflects their relationship and experiences.
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.
In the fanfic, their relationship could be developed by putting them in situations where they have to make sacrifices for each other. Maybe Finn has to give up a chance at a great treasure to save Huntress Wizard, or Huntress Wizard risks her magic to protect Finn. These selfless acts can make them realize how much they mean to each other. Also, they could have simple, everyday interactions like cooking together or sharing stories around a campfire that slowly build a bond between them.
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 might initially go into denial. He could keep expecting Jake to just pop back up like he always does. Then he'd probably get angry, at himself or at whatever caused Jake's death. Eventually, he'd start to accept it and try to honor Jake's memory by being a better hero.
You can develop their relationship by exploring their different perspectives. Ahsoka is younger and more idealistic in some ways, while Anakin has seen more of the galaxy's harsh realities. By having them debate about Jedi teachings, or how to deal with the Sith, their relationship can grow as they learn from each other. For instance, Ahsoka might suggest a more peaceful solution to a problem, and Anakin could initially be skeptical but then come around to see her point.
Well, perhaps in the fanfic, Jake's death could lead to a change in the balance of power in Ooo. Evil forces that were previously held at bay by Jake's strength start to emerge. Maybe the Ice King, instead of seeing Jake as an obstacle to his schemes regarding Princess Bubblegum, now has free rein. Finn would have to step up in a big way, but without his best friend, it would be extremely difficult. He may struggle with feelings of guilt, thinking he could have saved Jake.
Their relationship can be developed through shared musical experiences. For example, when they write a song together that really expresses their feelings for each other, even if they don't realize it at first. Also, having them support each other during tough times in the band, like when they face a bad review or a difficult gig.