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.
In Naruto x Final Fantasy 14 fanfiction, a great concept is the fusion of the two distinct magic systems. Naruto has its own form of magic with jutsu, while Final Fantasy 14 has a more traditional RPG - style magic. Seeing how characters would adapt to the other's magic could be fascinating. For instance, a Final Fantasy 14 character might struggle to control chakra at first but then find unique ways to incorporate it into their existing abilities. Additionally, the cultural differences between the two settings could lead to interesting storylines. The samurai in FF14 might interact with the samurai - like characters in Naruto, sharing techniques or having duels to prove their worth.