Well, you could write a fanfic where the Adventure Inc team gets separated in a large, maze - like underground cavern. Each member has to use their individual skills to find their way back to the others while also uncovering secrets of the place. This would add a lot of tension and character development as they face their own challenges alone.
One idea could be that Breezy gets lost in the Land of Ooo and Finn and Jake have to help her find her way back home. Maybe she has a special mission that she needs to complete but can't without their help.
One idea could be that the Water Princess discovers a hidden underwater kingdom that was once part of her domain but has been lost. Another could be her facing a drought in the Land of Ooo and having to use her powers in new and creative ways to bring water back. Maybe she has to team up with Finn and Jake to fight an enemy that is polluting the water sources.
One idea could be an OC with a unique Stand ability that is related to time manipulation, but in a different way than Dio's or Giorno's. For example, the OC's Stand can create time loops within a small area, trapping enemies. Another idea is an OC who is a descendant of a minor character from the original series and has a vendetta against one of the main villains because of something that happened in the past. A third idea could be an OC with a Stand that can transform into different animals, each with their own special abilities, and they use this to navigate the strange and dangerous world of Jojo.
One plot idea could be that Finn and Jake from Adventure Time end up in the Fallout wasteland. Finn, with his sword skills, could be on a quest to find a special artifact to get back to their own world. Jake, with his shape - shifting abilities, might help them survive against the various mutated creatures in the Fallout universe. Another idea is that the Vault - Tec technology in Fallout somehow merges with the magical elements of Adventure Time, creating strange new hybrids of mutants and magical beings.
One idea could be Finn and Jake swapping bodies. Finn in Jake's body might struggle with Jake's more laid - back and sometimes lazy attitude towards things like guarding the Tree Fort. He could be shocked at how comfortable Jake is just lying around. Another plot could involve Princess Bubblegum and Marceline swapping. Princess Bubblegum, in Marceline's body, has to deal with Marceline's vampire instincts like craving blood (in a non - too - gory way, of course). This could lead to some comical situations as she tries to resist her new urges while Marceline, in Princess Bubblegum's body, has to handle the responsibilities of ruling the Candy Kingdom and might be a bit too carefree for the Candy People's liking.
Maybe Princess Bubblegum creates a love potion to make Marceline fall in love with her, but things go wrong and it makes all the Candy Kingdom citizens fall in love with each other in a chaotic way. This could lead to a lot of humorous and heart - warming moments as they all try to sort out their new feelings.
You could write a fanfic about how the Mushroom War affected the magical elements in the Adventure Time world. Maybe the war disrupted the balance of magic, and certain characters are on a quest to restore it. They might encounter strange new magical creatures or phenomena that were born out of the chaos of the war.
One plot idea could be Finn and Huntress Wizard going on a quest to find a magical artifact that was lost in the Enchiridion. They face various challenges along the way, like battling strange creatures and solving riddles.
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 idea could be a crossover with another popular anime like Naruto. The JoJo characters could be transported to the Naruto world and have to deal with things like chakra and ninja techniques. They might use their stands in unique ways to combat the ninja threats. For example, Star Platinum could be used to quickly dodge and counter attacks from ninjas. Another plot could involve a crossover with the Fate series. The JoJo characters could be summoned as servants, and their stands could be seen as a form of their special abilities in this new context. This would create an interesting dynamic as they fight in the Holy Grail War.