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.
One great fanfic in this category is 'The Geass Exchange'. It explores an alternate scenario where Lelouch gives his Geass to a different character, Suzaku. This creates a whole new dynamic as Suzaku, with his own set of beliefs and moral code, has to grapple with the power of Geass. The story delves deep into how the Geass affects his relationships, not just with Lelouch, but also with other characters in the Code Geass universe like Euphemia. It's a really interesting take on the concept of Geass transfer and how it can change the course of events.
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.