Yes, it is. As a 'Code Geass' fan, it gives you more content related to the series. You get to explore the story further and might discover new aspects of the characters and the world.
Code Geass did start as a manga. However, its anime version became even more widely known and beloved. The manga provided the foundation for the rich story and characters that were further developed in the anime.
Code Geass started as an anime. The manga was based on the anime's success and came after to expand the story for fans. Some elements might differ between the two, but the anime was the original form.
A fanfic about Cornelia could be a character study. It could delve into her past, like her upbringing in the imperial family and how that shaped her into the strong - willed person she is. Maybe it could also show her softer side that she doesn't often display in the anime.