Their relationship could be developed through music. Marceline could teach PB how to play the guitar, and in the process, they open up to each other about their past traumas. PB might share her insecurities about ruling the Candy Kingdom, and Marceline could talk about her loneliness as a vampire.
They could develop their relationship by having to take care of a magical creature together. Maybe it's a baby dragon that they find and they both have different ideas on how to raise it. Through the arguments and compromises, they grow closer. For example, PB wants to feed it only candy while Marceline thinks it needs a more balanced diet of blood and regular food.
In fanfiction, their relationship could be developed by having them teach each other things. Finn could teach Marceline about his simple but effective survival skills, like building shelters or finding fresh water. Marceline, on the other hand, could teach Finn more about the complex magic that exists in the Land of Ooo. This exchange of knowledge would not only bring them closer but also make them respect each other more. Also, they could get into some light - hearted arguments about different ways of doing things, which would then lead to a better understanding of each other's personalities.
One idea could be that PB accidentally creates a magical device that swaps their personalities. Marceline then has to deal with being super sweet and girly while PB tries to maintain her tough rocker image. It would lead to a lot of comical situations as they try to navigate their daily lives in each other's shoes.
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.
Their friendship could be developed by having them share secrets. For instance, Marceline could tell Jake about a time she was really scared as a vampire. Jake, in turn, could share his fears about losing his family. This would deepen their bond.
Their relationship could develop uniquely if Rachel takes on a teaching role for the younger glee club members under Will's guidance. This role reversal would force them to see each other in new lights and create a deeper bond.
A unique way to develop their relationship could be through a series of challenges. For example, they are trapped in a mysterious dimension together and have to rely on each other to escape. In the process, they discover each other's strengths and weaknesses, and Kurama realizes that Kushina is not just Naruto's mother but a powerful ninja in her own right. This mutual respect and reliance would strengthen their relationship.
Their relationship could be developed uniquely by having them start as enemies in a sense. For example, Sephiroth could be on a mission that Aerith unknowingly gets in the way of. But instead of killing her immediately, he's intrigued by her. This initial encounter could spark a series of events where they keep crossing paths. Each time, they learn a bit more about each other. Maybe Aerith starts to see the pain behind Sephiroth's cold exterior, and Sephiroth begins to respect Aerith's strength and determination. Over time, this could lead to a complex relationship that is part friendship, part something more, as they both struggle with their own destinies and how the other fits into it.
Another way to develop their relationship uniquely is if Aipom gets a prophecy about a great danger that Ash will face in the future. Aipom decides to secretly train itself extra hard to be able to protect Ash. When Ash finally discovers what Aipom has been doing, he is touched and their relationship becomes even stronger. This would show a different side of their relationship, with Aipom being the protector instead of the one usually being protected by Ash.
Their relationship could be developed through a shared love of transfiguration. Hermione could seek out McGonagall for extra lessons and in the process, they bond over the beauty and complexity of the subject.
One way would be if they were both transported to a different dimension. In this strange new place, they have only each other to rely on. They would need to figure out how to survive and find a way back home. This shared struggle would bring out different sides of their personalities and force them to trust each other completely, thus developing their relationship in a very distinct way.