One idea could be a crossover with Steven Universe. Finn and Steven could team up to save both their worlds from a common threat. They both have a strong sense of justice and adventure, so it would be exciting to see them interact. Another could be a crossover with Gravity Falls. Dipper and Finn could go on a mystery - solving adventure together. The different magical elements of both shows would blend well.
One idea could be that Naruto and Finn the Human accidentally cross paths in a strange dimension. Naruto could be on a mission to find a new type of chakra source and Finn is exploring the unknown. They initially misunderstand each other and fight. But then they realize they both have a strong sense of justice and team up to defeat an evil force that threatens both their worlds.
First, choose the two (or more) universes you want to cross. Then, think about how the characters from each would interact. For example, if crossing with Teen Titans, consider how Finn would react to Robin's leadership style. Next, come up with a basic plot. It could be something like a new villain that threatens both worlds. Start with a strong opening that grabs the reader's attention, like Finn waking up in a strange new place that's part of the other universe.
In the original Adventure Time, Finn's story is about his journey through Ooo, facing monsters and solving mysteries. However, in Finn harem fanfiction, the story often revolves around his interactions with a group of characters in a more relationship - centered way. For example, in the show, Finn might have a brief encounter with a character, but in the fanfiction, that encounter could develop into a complex relationship dynamic within the 'harem'. Also, the fanfiction may introduce new character traits or backstories for Finn that are not in the original show to better fit the harem concept.
A plot could involve the Danganronpa characters being transported to the Land of Ooo. The students would have to adapt to the strange and magical environment. For example, Mondo Owada might get into a motorcycle race with Finn. Meanwhile, the mystery - solving skills of the Danganronpa characters could be used to solve some of the mysteries in the Land of Ooo, like the origin of the Ice King.
Well, Sakura's super strength and medical ninjutsu could be really interesting when combined with Princess Bubblegum's scientific knowledge. Sakura could help Princess Bubblegum in creating new types of medical treatments for the creatures in Ooo. And Princess Bubblegum could use her knowledge to enhance Sakura's medical ninjutsu tools. Also, Sakura's strength could be used to protect the Candy Kingdom from any physical threats, while Bubblegum uses her intelligence to plan defenses.
A really cool plot could be a race against time. The Ice King in Adventure Time has accidentally unleashed a time - based magic that is starting to freeze both his world and Naruto's world slowly. Naruto, with his speed and ability to adapt, teams up with Adventure Time characters like Marceline who has her own time - related powers to stop this and save both universes.
Well, one idea could be a fanfic that explores the backstories of the main characters in Don Regular Show and how it shaped their present. Another could be a crossover fanfic where they interact with characters from another popular show.