A more complex story could be that there is a prophecy in both universes that only Naruto and Ashe together can fulfill. This prophecy is related to saving all the worlds from a great darkness. They initially don't trust each other, but as they go on a journey to understand the prophecy better, they start to respect and rely on one another. Along the way, they encounter different characters from both universes who either help or hinder their progress.
One idea could be that Naruto is somehow transported to the League of Legends universe where he meets Ashe. They could team up to fight a common enemy, like an evil sorcerer who is threatening both the Hidden Leaf Village and Ashe's tribe. Naruto's ninja skills combined with Ashe's archery and leadership would make for an exciting adventure.
A fanfiction could focus on a power struggle between Ashe and Basch. Maybe there's a powerful artifact that grants great power, and both have different ideas on how it should be used. This could lead to an interesting conflict and character development as they each try to persuade the other to their side.
A great Ashe and McCree fanfiction could be set in a different world entirely. Say, a steampunk version of their world. Ashe could be a steampunk princess leading a rebellion, and McCree could be a wandering gunslinger who joins her cause. Their relationship could develop as they fight for justice in this new and strange world.
Another idea is that they are forced to team up to stop a dark cult that is trying to resurrect an ancient evil. Vaan and Ashe have to travel to different parts of the world to gather allies and resources. Along the journey, they discover secrets about each other's pasts that strengthen their bond and make them more determined to succeed.
One idea could be a what - if scenario where Naruto becomes a rogue ninja. This would completely change the dynamics of the story and his relationships. For example, he might be hunted by his former friends, and we could explore his inner struggle and reasons for turning rogue.
One idea could be to explore Naruto's life if he had been trained by a different sensei from the start. For example, if he was trained by Kakashi in a more intensive and specialized way, his skills might have developed differently. Another idea is to write about Naruto's life during a time skip where he goes on a secret mission that changes his perspective on the ninja world. Or you could focus on Naruto's relationships, like if he had a falling out with Sasuke much earlier and how that would have affected his growth as a ninja and a person.
One interesting idea could be the combination of Naruto's ninja skills with mecha technology. For example, Naruto might use his chakra to power up the mecha in unique ways. Another idea could be the new enemies that are created specifically for this mecha - Naruto concept, like robotic shinobi that require different combat strategies compared to normal foes. Also, the design of the mecha Naruto itself can be really cool, perhaps with features inspired by the Nine - Tails.
One idea could be that Krampus is summoned in the Naruto world during a particularly harsh winter. He mistakes the ninja villages for places full of naughty 'children' (in this case, ninjas who have misbehaved according to some warped view). Naruto then has to convince Krampus that the ninjas are actually good at heart and are just trying to protect their homes.
One idea could be that Naruto is somehow transported to the world of Athena. He might learn about her powers and the Greek mythology while also sharing his ninja skills. Maybe he could help Athena in a battle against a powerful enemy in her realm using his unique jutsus.
One idea could be a crossover where Naruto characters gain access to Halo technology. For example, Naruto might use a Spartan's energy shield in battle. Another could be a story where the Covenant discovers the ninja world and tries to conquer it, leading to an epic battle between ninjas and aliens.
In 'naruto omake fanfiction', a great idea would be to create an omake around the holidays in the Naruto universe. Picture Naruto trying to organize a Christmas - like celebration in the Hidden Leaf, with his own unique ninja - style decorations. Or an omake where during a festival, Naruto enters a eating competition against Choji and the comical struggle that follows. Also, an omake about Naruto teaching the younger generation some of his old pranks and the mayhem it causes.