A cool concept for a 'naruto fate crossover fanfic' could be the appearance of the Akatsuki in the Fate world. They might be after the Holy Grail for their own purposes, like using it to gain ultimate power or to bring about a new world order. Pain could use his Rinnegan abilities in ways that would surprise the Fate characters. Also, the concept of ninjas from Naruto world learning about the different classes of Servants in Fate, like Saber, Archer etc., and how they would strategize against them.
Well, you could have a story where the Jinchuriki from Naruto are somehow connected to the Servants in Fate. Maybe the tailed beasts have a similar origin to the powerful entities that become Servants. For instance, Kurama could have a past link to a powerful demonic - like Servant. And then Naruto, as the host of Kurama, would have to deal with this new revelation while also being in the middle of the chaos of the Fate world. It could lead to some really interesting battles and character development.
One idea could be to have the Servants from Fate interact with the Devils in DxD in a battle royale setting. Maybe the Holy Grail somehow ends up in the DxD universe and all the factions are after it.
One idea could be that Naruto is summoned into the Fate universe during the Holy Grail War. He might be mistaken for a Servant at first. His unique ninja abilities would disrupt the normal flow of the war. For example, his Shadow Clone Jutsu could be used to confuse the enemy Masters and Servants.
One idea could be that the Servants from Fate end up in the RWBY world and have to team up with the RWBY characters to fight a new common enemy. For example, a powerful Grimm that has some sort of magic ability similar to a Servant's Noble Phantasm. Another plot could involve the RWBY characters being summoned into the Fate universe and having to participate in the Holy Grail War, but with their own unique skills and abilities that disrupt the normal flow of the war. A third idea might be that a mysterious force starts fusing the two worlds together, and the characters from both have to figure out how to stop it while also dealing with the cultural and power differences between them.
For 'fate crossover fanfiction', you could consider a crossover with 'One Piece'. The Servants might be pirates or marines, sailing the Grand Line. A 'Fate' and 'Fullmetal Alchemist' crossover would also be cool. Picture Servants trying to solve the mysteries of alchemy. There's also potential in a 'Fate' and 'My Hero Academia' crossover, with Servants having Quirks.
One idea could be that the characters from Teen Wolf end up in the Fate universe and have to deal with the complex magic system there. For example, Scott could try to use his werewolf abilities to control the powerful magical forces in Fate. Another could be a love story where a character from Teen Wolf falls in love with a Fate character, like Stiles falling for a Fate mage. Maybe the Teen Wolf pack has to fight against a common enemy in the Fate world, something that combines the bestial nature of the werewolves with the magical threats of Fate.
One interesting plot idea could be that the Servants from Fate Stay Night end up in the Naruto world. For example, Saber could team up with Naruto. Saber's noble and just nature would blend well with Naruto's determination to protect his village. They could face off against common enemies like Orochimaru, who might be interested in the power of the Servants. Another idea is that the concept of chakra in the Naruto world could interact with the magic in Fate Stay Night. Maybe a mage from the Fate universe tries to study chakra and finds a way to combine it with their own magic system, creating a whole new type of power. A third plot could involve the Holy Grail War taking place in the Hidden Leaf Village. The ninjas would be both intrigued and cautious about this strange new event, and the Servants would have to navigate the politics and unique environment of the village while fighting for the Grail.
A Naruto x One Piece crossover would also be cool. Naruto could end up on the Grand Line and team up with the Straw Hat Pirates. His ability to create shadow clones could be useful in battles against other pirates, and he might learn some new skills from Luffy and the crew.
A third idea could be a crossover with a dystopian empire. The shinobi would face a much harsher and more controlling regime. This could lead to some really intense storylines as they try to either overthrow the regime or find a way to co - exist within it.
Perhaps there could be a common enemy that both Inuyasha and Naruto have to face. A powerful demon or a rogue ninja who has found a way to travel between their worlds. They would have to combine their unique skillsets, like Inuyasha's speed and strength along with Naruto's Rasengan and his ability to inspire others. Their friends would also play important roles, like Sakura helping with medical needs and Kagome providing spiritual support.
One idea could be to have Naruto train with characters from other popular shonen series like Goku from Dragon Ball. They could exchange training techniques, for example, Naruto teaching Goku about chakra control while Goku shows Naruto how to better use his physical strength. Another idea is to have Naruto cross over with characters from Fairy Tail. The Fairy Tail mages could introduce Naruto to different types of magic-based training that he could incorporate into his ninja training. A third idea would be to have Naruto train with characters from One Piece. They could have a unique training session on a deserted island, learning survival skills from each other.