A possible plot could be that the tailed beasts in Naruto are somehow related to the Astrals in Final Fantasy 15. The characters from both universes come together to figure out this connection. For example, Sasuke could be intrigued by the power of the Kingsglaive in Final Fantasy 15 and try to learn their combat styles. Meanwhile, Prompto could be fascinated by Naruto's ability to create shadow clones and try to incorporate that into his own fighting techniques.
One interesting plot could be that Naruto and his friends somehow get transported to the world of Final Fantasy 15. Naruto's ninja skills would be a huge asset in the new world. He could team up with Noctis and his crew to fight against a common enemy, like a powerful daemon that is causing chaos in both their worlds combined. Maybe Naruto could teach Noctis some of his chakra control techniques in exchange for learning some of the magic from Final Fantasy 15.
Well, it could be a plot where the Akatsuki from Naruto are after the crystal in Final Fantasy 15. Noctis and his friends have to protect it with the help of Naruto and his comrades. The Akatsuki's unique abilities would pose a great threat to the world of Final Fantasy 15. Naruto's rasengan could be a powerful weapon against the Akatsuki in this new setting, and the characters from Final Fantasy 15 could learn from Naruto's experience in dealing with such dangerous groups.
One possible plot could be that Naruto is somehow transported to the world of Final Fantasy IV. He teams up with Cecil and the others. Naruto's unique ninja skills could be used to help in battles against the various foes in FFIV's world, like the Golbez's minions. His rasengan could be a powerful new attack for the group.
In this fanfiction, it could be that there is a prophecy that involves both Lulu and Naruto. The prophecy states that only by combining their powers can they save both their universes from a great evil. So, they meet and at first have communication problems due to their different backgrounds. However, as they train together and share their stories, they form a deep bond and work towards fulfilling the prophecy.
Perhaps in the fanfiction, the Akatsuki organization somehow gets hold of the power to summon Final Fantasy creatures. Naruto then has to figure out how they got this power and stop them. He might have to train hard to master the same summoning techniques. He could end up summoning a Shiva to counter the Akatsuki's Ifrit summon in a huge battle that shakes the very foundations of the ninja world.
Well, it could be about Naruto characters getting sucked into the world of Final Fantasy 8. Maybe Naruto could team up with Squall. They'd fight some of the FF8 monsters together, and Naruto could use his jutsus while Squall uses his gunblade. It'd be a really cool mix of the two universes.
One interesting concept could be the combination of Naruto's ninja abilities with the job classes in Final Fantasy 14. For example, a character from Naruto might take on the role of a Black Mage in FF14, using their chakra in a way similar to how a Black Mage uses mana. Another concept could be the interaction between the different worlds. Maybe the Hidden Villages could exist in the same universe as Eorzea, and there could be diplomatic relations or even battles between the two. Also, the idea of characters from FF14 learning ninja techniques from Naruto characters would be really cool.
Some cool crossovers might include Naruto's summoning jutsu being used in the world of Final Fantasy X - 2. He could summon giant toads or snakes, which would be a strange sight in Spira. The Al Bhed language from Final Fantasy X - 2 could be something Naruto characters have to learn. And perhaps the Farplane in Final Fantasy X - 2 could have a connection to the afterlife in the Naruto universe. This could lead to some really deep and emotional storylines where characters from both worlds interact with the concept of death and the afterlife.
In some Naruto x Final Fantasy fanfiction, the summons in Naruto could be related to Final Fantasy summons. Like the toads in Naruto could have a connection to a similar creature from Final Fantasy. This would create unique interactions between the two universes.
One interesting plot could be that Naruto is somehow transported to the X - Men universe. He teams up with Wolverine. Naruto's ability to create shadow clones could be used in interesting ways during battles, like confusing the enemies. Wolverine could teach Naruto some combat skills from the X - Men's fighting style. They could go on a mission to stop a powerful mutant villain who has plans to take over the world, using Naruto's chakra powers and Wolverine's adamantium claws.
One possible plot could be that Naruto is somehow transported to the X - Men universe. He teams up with Wolverine. Naruto's unique ninja skills, like his shadow clones, could be used in combat against the Brotherhood of Mutants. Wolverine could teach Naruto about the concept of mutants and their struggles, and Naruto could in turn show Wolverine some of his ninja wisdom.
One possible plot could be that Naruto is somehow transported into the world of Final Fantasy. He meets Lightning and they team up to fight a common enemy. Maybe a powerful sorcerer who is threatening to destroy both their worlds. Naruto's ninja skills combined with Lightning's combat abilities would make for an interesting dynamic.