One idea could be Naruto being summoned as a Servant in the Fate Zero world. His unique ninja abilities would make for an interesting dynamic against the traditional Servants. He could use his shadow clones to outnumber opponents or his Rasengan in combat.
Perhaps a scenario where the concept of chakra is somehow linked to the Holy Grail War in Fate Zero. Naruto might be on a quest to understand this new type of energy system while also getting involved in the battles between the mages and their Servants. He could form unexpected alliances with some of the characters, like Kiritsugu, based on their shared sense of justice.
One idea could be to have the Servants from Fate Grand Order interact with the events and characters of Fate Zero in unexpected ways. For example, Mash could end up in the middle of the Fourth Holy Grail War and try to prevent some of the tragedies from happening. She might team up with a young Kiritsugu, using her knowledge from the future to guide him. Another could be that a Grand Order Servant like Artoria Alter gets involved in the war and changes the power dynamics completely, as her dark and powerful nature would be a wild card in the already complex Fate Zero setting.
One plot idea could be that Naruto is summoned into the Fate Grand Order universe. He teams up with Mash to fight against the various Servants who have gone berserk. Naruto's unique jutsu like Rasengan could be a new and powerful attack in that world.
One idea could be to create a crossover where Naruto characters are transported to the world of Familiar of Zero. For example, Naruto could be mistaken for a powerful familiar. Another idea is to have a character from Familiar of Zero learn ninjutsu from Naruto. Maybe a familiar in the Familiar of Zero world could have the Nine - Tails' chakra sealed within it.
Naruto and Saber would be an interesting pairing. Saber's noble and chivalrous nature combined with Naruto's energetic and optimistic personality could create a unique dynamic. They could learn from each other's fighting styles and moral codes.
One idea could be a crossover of Naruto's world and the Familar of Zero world where Naruto has to learn the magic system in the new world. Maybe he teams up with Saito from Familar of Zero to fight a common enemy.
Perhaps in the fanfiction, the Zero Tails is a manifestation of the negative emotions of the ninja world as a whole. A group of young ninjas discover this and they try to find a way to purify the Zero Tails. They seek the help of old sages and learn ancient jutsu to attempt this purification, which could lead to a battle against those who want to keep the Zero Tails as a weapon of mass destruction.
One idea could be a crossover where Naruto characters gain the abilities of Mega Man Zero. For example, Naruto could use Zero's saber in combat, adding a new dimension to his fighting style. Another idea is a story where the world of Naruto is invaded by enemies from the Mega Man Zero universe, and the ninja have to adapt quickly to survive. Maybe even a story where Mega Man Zero is somehow transported to the Naruto world and has to learn about chakra and ninja techniques to find his way back home.
One idea could be exploring new ninja techniques not seen in the original Naruto series. For example, a unique combination of elemental jutsus created by a character in 'naruto ruler zero fanfiction'.
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 interesting crossover could be a story where Sakura from Naruto gains the power of a Servant from Fate Zero. It would be really cool to see her use those new abilities in the Naruto universe. For example, she could use a powerful Noble Phantasm to fight against powerful enemies in Konoha.