I don't know. Fanfiction creation is quite diverse, and the basis for 'The Idea of You' could be anything from another story to a unique concept in the author's mind.
One common theme is 'alternate universes'. Writers like to imagine the characters in different settings, like a modern - day version of a historical character. Another theme is 'romance'. Many fanfictions explore relationships between characters that were not fully developed in the original work. Also, 'power - up' is a theme where a character gains new abilities or powers not present in the original.