One possible fanfiction could be about Naruto characters finding themselves in the Avatar world. For example, Naruto's determination and never - give - up attitude could be a great addition to the Avatar universe. He might learn bending techniques in a unique way, perhaps by combining his chakra control with the flow of chi in the Avatar world. This could lead to some really interesting battles and character interactions.
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.
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.
Naruto and Jake would be a really fun pairing. Naruto's high - energy and determination combined with Jake's laid - back yet powerful nature would create some great interactions. Jake could learn about Naruto's dreams and Naruto could be amazed by Jake's shape - shifting abilities.
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 common theme is the clash of cultures. Naruto comes from a world of ninjas and strict training, while Adventure Time has its own very different and often wacky culture. Fanfictions often explore how Naruto adapts to this new world and how the characters from Adventure Time react to his presence.
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.
One idea could be to have Naruto find the Ocarina of Time in the Hidden Leaf Village. Maybe it was left there by a wandering traveler from the 'Ocarina of Time' world. Naruto, being curious, starts to play it and unknowingly opens a portal to the world of 'Ocarina of Time'. He then has to navigate this new world, using his ninja skills and making new friends and allies there.
Another possibility is that Adventure Time's Princess Bubblegum could collaborate with Sakura. Princess Bubblegum's scientific knowledge and Sakura's medical and combat skills could lead to some amazing adventures. For example, they could be working together to heal a strange illness that is affecting both the Land of Ooo and the Hidden Leaf Village.