One possible plot could be that Waver uses his intelligence to outwit the more experienced mages. He might form unexpected alliances, like with some of the Servants who see his potential. For example, he could team up with Rider to not just fight in the war but also to learn from him, growing as a mage and a person throughout the events of the Grail War.
A great plot could involve Waver starting off as an underdog. Everyone underestimates him in the 5th Holy Grail War. But he manages to find a loophole in the rules of the war. Using this, he starts to turn the tables on his opponents. He could also find a way to communicate with the Grail itself in an unorthodox manner, learning secrets about the war and using that knowledge to his advantage. His journey could be about not just winning the Grail but also about changing the perception of what it means to be a mage in the context of the Grail War.
Kotomine Kirei could be one of his main rivals. Kirei has a complex and devious nature, and his experience in the Grail War would make him a formidable opponent for Waver. Another could be Tohsaka Rin. She is a skilled mage, and her determination to win the Grail would put her at odds with Waver.
A really cool plot for a Naruto Holy Grail War fanfiction could be that the Holy Grail itself has some connection to the Sage of Six Paths. The characters would then be fighting not just for the Grail's power, but also to uncover the secrets it holds about the origin of ninjutsu. The Servants could have unique abilities based on the different elements and kekkei genkai in the Naruto world. For instance, a Servant with the Byakugan could have enhanced perception and long - range attack abilities as part of their Servant skills. The story could also explore how the rules of the Holy Grail War are affected by the different values and beliefs in the Naruto universe.
One possible plot could be Archer being a wildcard in the war. He might start off allied with one of the traditional masters, but then discovers a hidden agenda within the Holy Grail War itself. So, he decides to break away and fight on his own terms to expose the truth behind the war.
Perhaps the story focuses on a previously unknown Master. This Master could be a young mage who accidentally stumbles upon the Grail War and has to quickly learn the ropes. They might form an unexpected bond with their Servant, like a Servant who is usually seen as cold-hearted but warms up to this inexperienced Master. This new dynamic could lead to a whole new set of adventures and challenges in the Grail War.
One idea could be introducing a new type of Servant, like a Servant from a different mythology not previously seen in the Grail Wars. This would bring in new powers and dynamics. Another could be having a master who is an ordinary person thrust into the war by accident and has to quickly learn the ropes. For example, a school student who stumbles upon the Grail War's secrets and has to fight to survive.
One possible plot could be that with 49 servants, the Grail War becomes extremely chaotic. There could be multiple alliances and betrayals among the masters and servants. Maybe some servants with unique abilities form a group to try and change the rules of the Grail War, as they realize that with so many participants, the traditional way of winning the Grail might lead to too much destruction.
One could be a young female mage as the main character. She might be from a family that has been on the outskirts of the mage society and is trying to prove herself in the Grail War.
The plot could involve Emiya's discovery of secrets within the Grail War. He might find out that the Grail has been corrupted in ways not known in the original story. This could lead him on a quest to either purify the Grail or stop the war altogether. Also, his relationship with other Servants like Saber could be different. Instead of the typical rivalry, they could develop a mutual respect and a friendship that transcends the war. Emiya might also use his unique skills and abilities in new and creative ways. He could combine his projection magic with some other power he discovers during the war to gain an edge against his opponents.
If Naruto is summoned in the Holy Grail War fanfic, he would bring his unique ninja skills. His shadow clone jutsu could be used to confuse opponents and gather information. Also, his Rasengan could be a powerful offensive move against other Servants. He might have a hard time understanding the rules at first, but his determination would make him adapt quickly.
One interesting concept is alternative Servant summonings. For example, seeing a different class for a well - known character like if King Arthur was summoned as a Caster instead of a Saber. Another concept could be different Masters, perhaps a non - traditional or more morally ambiguous Master. And then there are the alternate storylines that might change the location or rules of the Holy Grail War.