One interesting concept could be the exploration of different kingdoms in more detail. Maybe a fanfiction could focus on the unique cultures and inhabitants of each kingdom that weren't fully explored in the original work.
Arthur, without a doubt, is often a very popular character. He's the main character in the original, so many fanfictions will center around him, his adventures, and his growth throughout the story.
I'm not entirely sure as I haven't delved deeply into this specific 'Keys to the Heart' true story. It could be about a journey of self - discovery, or perhaps a story of relationships and how they unlock certain aspects of one's heart.
Yes, 'Kingdom of Heaven' is based on some historical events. It depicts the Crusades era, showing aspects of the conflict between Christians and Muslims in the Holy Land. However, it also takes some creative liberties for the sake of storytelling.