An 'OC' (original character) fanfic in the 'Code Geass' universe centered around the emperor can be really interesting. It might explore the emperor's backstory in more depth, perhaps from a different perspective than the original series. For example, it could show his early life and how he came to hold such extreme views in the series.
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.
The fanfic might also be unique in how it interprets the emperor's character. Maybe it shows a more human side of him that was not fully explored in the original series. The OC could be the key to unlocking this new view, through their relationship or encounters with the emperor.
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.