One idea could be to focus on a side character from the series. For example, a young squire who dreams of becoming a great knight. He could get caught in the middle of a political intrigue between two lords that is related to the main war in Grancrest War. His journey to prove himself and survive in this complex situation could make for an interesting fanfiction.
The Record of Grancrest War fanfic is quite interesting. It often takes the existing plotlines and twists them in new and exciting ways. Some fanfics focus on the relationships between the different lords and mages, which is a great aspect to explore as it was a central part of the series. It gives a fresh perspective on the power dynamics in the story.
I like the way they portray Siluca in the fanfic. She is made even more intelligent and resourceful. Her strategies are more detailed and it shows her as a truly formidable character.
Power and politics are main themes. In the 'Record of Grancrest War Novel', different factions struggle for power, which is often related to political maneuvers.
First, give them clear goals. In Grancrest War, a character could be aiming to rise in the ranks of their military order. This goal will drive their actions and decisions. Second, give them flaws. A character who is overly confident in their combat skills might underestimate their opponents and get into trouble. Also, consider their backstory. If a character grew up in a war - torn village, it will shape their views on war and loyalty in the Grancrest War universe.
One of the main characters is Siluca. She is a mage and plays a crucial role in the story. Another important character is Theo. He is a young man with great leadership potential. And then there's Marrine, who is also involved in the political and military affairs of the story.
Many fanfictions might also explore different relationships. They could develop the romantic relationships between characters in new and interesting ways. Maybe there's a fanfiction where two secondary characters have a secret relationship that affects the overall plot in unexpected ways.