Another common plot is Finn getting turned into a vampire briefly. This creates a whole new dynamic in their relationship. Marceline has to teach Finn how to control his new vampiric urges while also dealing with her own feelings about sharing her 'vampire world' with Finn. It often leads to some really interesting and tense moments between them.
There are also fanfictions that put Flame Princess in alternate universes. In these stories, she might be in a world where magic works differently or where the rules of the Land of Ooo are completely changed. For instance, she could be in a world where she is the only one with fire powers and has to protect the entire universe from an ice - based villain. This allows for a lot of creativity and unique interpretations of her character.
One popular storyline could be Finn's hair having magical properties. For example, his hair might grow uncontrollably and start to have a life of its own, leading him on an adventure to find a way to control it. Another could be a story where Finn's hair is the key to unlocking a hidden realm. Writers might also explore a plot where Finn's hair changes color based on his emotions, which then affects his adventures in strange ways.
One popular storyline is about Marceline and Simon reminiscing about their past adventures. It often delves into the bittersweet memories they shared before Simon's transformation. Another is a what - if scenario where Simon finds a way to reverse his condition and how it would impact their relationship. There are also fanfictions that focus on their adventures in an alternate universe, facing new challenges together.
One popular storyline could be a character from our world getting transported into the Adventure Time universe and having to adapt to its wacky rules. For example, they might struggle with the magic and strange creatures. Another could be a main Adventure Time character being transported to an unknown realm and trying to find their way back home while facing new challenges. And there could also be a story where multiple characters are transported to a parallel Adventure Time world with a different set of power dynamics.
A great storyline could be Deadpool being sent to Ooo to retrieve a magical item that he thinks will make him even more indestructible. He teams up with Finn and Jake, but along the way, he starts to care about the people in Ooo more than the item. Another could be that the Lich somehow makes a deal with Deadpool's enemies in his own world and they all end up in Ooo, creating a huge battle. Deadpool has to choose between his old life and his new friends in Ooo.
Some writers develop their relationship by exploring their different personalities. Finn's optimistic and naive nature contrasts with Marceline's more cynical and world - weary self. By having them interact and learn from each other, the relationship grows. For instance, Finn can teach Marceline to see the good in things again, and Marceline can show Finn the harsher realities of life in a way that helps him mature.
One interesting storyline could be a Futa character going on a quest in the Land of Ooo, facing unique challenges that are different from the original Adventure Time plot. Maybe they have to use their special abilities to save a magical creature.
Perhaps Finn's death is an accident. He could be exploring an uncharted area of Ooo and stumble upon a dangerous trap or ancient magic that he doesn't understand. His death shocks everyone in Ooo, and his friends start to question the safety of their continued adventures. This could lead to a whole new set of adventures for his friends as they try to come to terms with his death and make Ooo a safer place in his memory.
One possible storyline could be Gumball and Marshall teaming up to solve a mystery in the Land of Ooo. Maybe there's a strange disappearance of a magical creature and their combined skills - Gumball's wits and Marshall's vampire powers - help them find out what happened.
Another possible storyline is the si gamer trying to adapt to the strange rules of the Adventure Time universe. Since it's a very different world compared to the ones in most games, they have to learn quickly how things work, like the way magic functions and how to interact with the various wacky creatures. This could lead to a lot of comical and interesting situations as they fumble through their new environment.