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.
A fanfiction could explore an alternate ending. What if the main characters made different choices? Maybe the alliance that was formed in the original story breaks apart earlier due to a misunderstanding. This would lead to different battles and power dynamics, and you could explore how the characters adapt or perish in this new scenario.