One idea could be to have Naruto characters in the world of Final Fantasy 15. For example, Naruto could be a guest in Noctis' group. He could use his ninja skills like shadow clones to help in battles against the daemons in Final Fantasy 15. His chakra could be seen as a new form of magic in that world.
Another possibility is a story where the power of the crystals in Final Fantasy 15 has some connection to the tailed beasts in Naruto. Maybe the crystals' energy can be used to either control or enhance the tailed beasts. And characters from both universes team up to protect or fight over these powerful elements.
A possible fanfiction could involve a fusion of the two worlds' power systems. The characters from both Naruto and Final Fantasy 13 could exchange knowledge about their abilities. For instance, the summonings in Naruto could be related to the Eidolons in Final Fantasy 13, and they could find ways to combine their powers for a greater good.
One idea could be to have Naruto characters in the world of Final Fantasy 10. For example, Naruto could be a Blitzball player, using his speed and agility. Sakura could be a healer, like a White Mage in Final Fantasy 10. They could be on a quest to stop Seymour from achieving his evil plans, with Naruto's Rasengan being a powerful attack against the fiends in Spira.
One idea could be to have the characters from RWBY end up in the world of Final Fantasy 15. For example, Ruby could meet Noctis and they could team up to fight some common enemy. Ruby's speed and scythe - wielding skills could be a great addition to Noctis' combat style.
Perhaps a Final Fantasy character gets transported to our modern world and has to work at a Lululemon store to earn money for their journey back home. They learn about modern fashion and the concept of athleisure, and there could be some comical moments as they try to adapt to this new environment.
One idea could be a crossover where Sephiroth somehow ends up in the wizarding world of Harry Potter. Maybe he's a dark force that Voldemort tries to use for his own gain, but Sephiroth has his own agenda. He could use his powerful magic, which is different from the magic in the Potterverse, to create chaos.
Well, it could be really cool. Picture Naruto in the world of Final Fantasy VII. He might use his ninja skills along with the materia system. For example, he could use a fire materia to enhance his fire - based jutsu. It would be a wild mix of two very different universes.
One idea could be to have characters from Naruto end up in the world of Danmachi. For example, Naruto himself could find his way into the dungeon and his ninja skills would be put to the test against the dungeon monsters. His shadow clone jutsu could be used in interesting ways, like sending clones into different dungeon levels to scout or fight. Another idea is to have the concept of chakra introduced into the Danmachi world. This could change the way adventurers fight and level up. Maybe some of the Naruto characters could train the Danmachi adventurers in new combat techniques.
One idea could be a crossover where Naruto is somehow transported to the Star Wars universe and meets Padme. Maybe he uses his ninja skills to protect her from the Sith.
Another concept is that Naruto is sent to Middle - earth. There, he could use his ninja skills and chakra powers. He and Gandalf could team up to fight Sauron. Naruto's ability to make friends and inspire others would be really useful in uniting the different races of Middle - earth against the common enemy.
A more complex idea is that the Matrix is used to create a virtual training ground for the shinobi. They enter the Matrix to practice their skills in a more controlled environment. However, they soon realize that there are some dark secrets within the Matrix, and Naruto has to lead the way to uncover the truth and free those who are trapped inside.