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.
One interesting Code Geass fanfic on TV Tropes might be those that explore alternate endings. For example, some fanfics might change the outcome of certain battles or character relationships. Another type could be ones that focus on the backstories of minor characters, which can give a fresh perspective on the Code Geass universe.
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.
The 'Code Geass 5 Year Fanfic' might expand on the original world - building by delving deeper into the politics of the different nations. For example, it could explore how Britannia's rule has changed over the five years, whether there are new rebellions brewing in different regions. It may also focus on the economic aspects. Maybe some regions are suffering more due to Britannia's policies, and this leads to new forms of resistance.