One great Naruto x Final Fantasy fanfiction is 'The Ninja of Spira'. It combines the unique ninjutsu skills from Naruto with the magic and world - building of Final Fantasy. The story has an interesting plot where Naruto characters are transported to the world of Final Fantasy. It's a great read if you like crossovers that blend action and magic.
A prevalent theme is also the interaction between different cultures. Naruto has its own ninja culture and Final Fantasy X has a very distinct world with different races and beliefs. Fanfictions often explore how the characters from these two different settings interact with and learn from each other's cultures. Also, the idea of friendship and teamwork is often emphasized, as characters from both series team up to overcome great threats that neither could handle alone. This can lead to some really interesting character development and storylines.
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.
Sasuke and Cloud is also a common pairing. Both are brooding and powerful characters. Fans are interested in seeing their clashing personalities and how they might influence each other, whether in a battle or in a more cooperative situation.
Well, sometimes they become friends. Like Tidus from Final Fantasy X might meet Naruto and they start training together. They could learn from each other's strengths, like Tidus teaching Naruto about blitzball moves that Naruto could then use in his own combat style.
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.
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.
The settings can be combined by having the hidden villages from Naruto be located in different parts of Spira. For example, Konoha could be in the middle of a forested area similar to Macalania Woods. The temples in Final Fantasy X - 2 could also serve as important locations in the Naruto - Final Fantasy X - 2 combined world. They could be places where chakra or magic is especially strong.
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 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.