A crossover might involve the storylines. Picture the ninja world of Naruto facing a threat similar to the fal'Cie in Final Fantasy XIII. The shinobi could team up with the characters from FF XIII to fight this common enemy. The different cultures and power systems would create a really engaging and unique story.
In terms of character relationships, Lightning from Final Fantasy XIII could train with Sasuke in Naruto. Lightning's combat skills and discipline could have an impact on Sasuke's development. Or perhaps Sakura could learn some healing magic from the characters in FF XIII, which would enhance her medical ninjutsu.
A cool crossover might involve the jutsu in Naruto being combined with the spells in Final Fantasy 14. Picture Sasuke using a spell like Blizzard III from Final Fantasy 14, but with his own Uchiha - style twist. Also, the ninja villages in Naruto could be compared to the city - states in Final Fantasy 14. The fanfiction could explore how the politics and power dynamics of both worlds interact when characters from each are forced to cooperate. Maybe the Hidden Leaf Village and Limsa Lominsa form an alliance to fight a common enemy that threatens both their existences.
One interesting crossover could be the characters from Naruto using the Materia from Final Fantasy 7. For example, Naruto with the Fire Materia could enhance his Rasengan with fire elemental powers. It would be a really cool visual to see.
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.
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.
Another plot could revolve around an alternate ending. Instead of the events that were perhaps expected to occur in the game, the fanfiction could take a different turn. For instance, if a character was supposed to die in the game, in the fanfiction they could survive and have a whole new role to play, leading to a completely different story arc for all the characters involved.
One interesting crossover could be the characters from Naruto having to deal with the complex summon system in Final Fantasy X. For example, Naruto might be intrigued by the Aeons and try to form some sort of bond with them. Another aspect could be the sharing of jutsu and magic. Maybe a character from Final Fantasy X learns how to use a chakra - based jutsu, or vice versa.
One good crossover could be a story where Naruto characters gain the power of the l'Cie from Final Fantasy 13. For example, Naruto could be a l'Cie with a focus on his wind - based jutsu, making it even more powerful. Another idea is a plot where the ninja world is in danger and the characters from Final Fantasy 13 come to help, bringing their unique magic and technology. It could also be interesting to see how the summons in Naruto compare to the Eidolons in Final Fantasy 13.
One interesting crossover could be the combination of RWBY's unique combat styles with Final Fantasy's magic systems. For example, Ruby's scythe - wielding skills might be enhanced with some of Final Fantasy's fire or ice magic spells. It would create really dynamic and visually stunning fight scenes.
One interesting crossover could be characters from Final Fantasy having to navigate the political intrigue of Westeros. For example, Cloud from Final Fantasy VII might find himself in King's Landing. His combat skills could be put to use in the battles for the Iron Throne. He'd probably be a bit confused by all the backstabbing and political games, as he's used to more straightforward battles against Sephiroth and the Shinra.
A great aspect of a Final Fantasy 10 and Kingdom Hearts fanfiction crossover could be the character relationships. Yuna and Sora, for instance, could form an interesting friendship. Yuna's calm and spiritual nature might contrast and complement Sora's more impulsive and adventurous side. They could learn from each other's experiences, with Yuna teaching Sora about the summoning and the fayth, while Sora shows Yuna the power of the Keyblade and the importance of believing in oneself in the face of great evil.