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.
A possible plot development is that Emiya gets involved in a love interest. In this fanfic, he could fall in love with a human or even another Servant. This would add a whole new layer of complexity to his character and his actions in the Grail War. His combat style might also evolve. He could learn new techniques from observing other Servants' battles. And his relationship with the Grail itself could be explored more deeply. He might try to understand the true nature of the Grail, whether it's a tool for wish - granting or something more sinister, and this exploration could drive the plot forward.
In this fanfic, Emiya might be more assertive. In the original, he is often reserved and follows his own code quietly. But here, perhaps the circumstances of the 4th Grail War force him to be more outspoken and take more initiative. He could be more open about his feelings and motives, which is different from his rather mysterious nature in the original. For example, he might directly tell other Servants his thoughts on the Grail War instead of keeping them to himself.
In a fanfic where Kiritsugu is summoned in the 5th Grail War, he would bring his own set of skills and motives. His skills as an assassin would come in handy. He might try to figure out the true nature of the Grail this time around, especially after his experiences in the previous wars. His relationship with other Servants could be complex. Some might respect him for his past achievements, while others might see him as a threat. And his actions would likely be guided by his own moral code, which, although not entirely conventional, has its own logic.
Another plot could involve Shirou getting a glimpse into the past of the Grail War through some sort of magical artifact. He discovers secrets that were long buried and this knowledge could either help him in the war or put him in greater danger as others may want to silence him.
A fanfic like this might start with Shiki's sudden summoning. He'd be confused at first, but then quickly adapt. His skills in combat, especially his ability to see the 'lines of death' on his opponents, would make him a formidable Master. He could encounter Servants from different mythologies and have to use his wits as much as his powers to survive in the war. For example, he might face a Servant with powerful magic and have to find a way to counter it using his unique perception.
Another plot idea might be to focus on Shirou's internal struggle during the Grail War in more depth. Show how his ideals are tested not just in battle, but also in his interactions with other Masters and Servants. Maybe he has to make some really tough moral choices that could change the course of the war.
Servants are also often main characters. For instance, Saber, with her noble and powerful presence. In the fanfic, she might have different relationships with other characters compared to the original. Her backstory and her connection to the Grail War are important elements that make her a main character.
The relationships between the characters can also be an interesting concept. In the fanfic, characters who may not have interacted much in the original could form strong alliances or rivalries. It can show how different personalities clash or complement each other in unexpected ways.
His abilities would be a game - changer. His strategic mind could outwit many of the other Masters. His use of modern weapons combined with his Servant's powers would be a deadly combination.
One interesting plot point could be Shiki's internal struggle. He has his own moral code which might conflict with the cut - throat nature of the Grail War. Another could be his relationship with his Servant. If he summons a Servant with a strong personality, there would be clashes and also moments of mutual understanding. For example, if he summons a heroic - type Servant who values honor, and Shiki has his own way of doing things.
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.