Perhaps there are also some characters who are in a moral gray area. They might not be clearly on the side of good or evil. For example, a mercenary who initially fights for money but then has a change of heart during the course of the holy war. These types of characters can add more depth and complexity to the story.
Shirou is obviously a main character. Often, Saber would still be a major part as well, since she has a strong connection with Shirou in the original story. Other Servants and Masters from the original Holy Grail War concept might also appear.
One of the main themes could be the struggle between good and evil. In many 'holy war' type of novels, there is a clear divide between the forces representing righteousness and those representing malevolence. Another theme might be the test of faith. The characters often face challenges that make them question or reaffirm their beliefs.
There were a few novels that met this requirement, such as Sword Immortal who joined the chat group, Infinity from the Giants, and The Final War of the Apocalypse. The protagonists of these novels were involved in the Holy Grail War, but the main content of the novels was not the Holy Grail War. It involved different worlds and storylines. If you're interested in these novels, you can read them. I hope you like my recommendations.
The Straw Hat Pirates would probably be the main characters. Luffy, with his determination and leadership, would be right in the middle of the action. Zoro, with his powerful swordsmanship, would be crucial in battles. Nami could use her knowledge of the weather to gain an advantage. And other members like Sanji, Usopp, and Chopper would also play important roles.
There are several main characters. Sigurd is a prominent one. He's known for his chivalry and military prowess. Then there's Seliph, who is related to Sigurd and has his own journey in the story. Deirdre also plays a vital role. She has a complex relationship with Sigurd and is involved in the overall plot twists.
Since I don't know the exact story, it's hard to say. But likely there would be masters, who are the ones controlling the servants in the Holy Grail War. And then there would be the servants, who might have various heroic or mythical identities.
The 'Holy War' cartoon's main theme usually involves a battle between opposing forces, with an underlying message about the importance of courage, sacrifice, and the pursuit of a greater good.
The plot of the manga is complex and full of action and adventure. The characters are diverse and have unique traits and abilities.
The Final Holy War was a movie from Ultraman Diga. The story took place after the final battle between the evil gods Catanjee and Tiga Ultraman. The human world welcomed two years of peace. In the film, Captain Irima followed the GTS archeological team to the ancient ruins of Luluye in the South Pacific. They once again discovered the destroyed super-ancient city and the stone statues of the three evil gods, Hitra. The movie showed the decisive battle between Ultraman Diga and the evil Ultraman, as well as the story of mankind facing the challenge and redemption of the dark forces. As for the specific plot and plot development of the movie, you can watch the movie or related videos to understand.