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.
First, study the existing world of Code Geass. Know the politics, the social hierarchy, and the technology. For example, if your OC is a Britannian, they should act according to the values and norms of that society. Second, give your OC a clear motivation. It could be revenge, love, or a desire for power. And third, don't make them overpowered. Even if they have a special ability, it should have limitations.
The original plans for the Code Geass story might have involved a broader scope of the geopolitical situation in the anime's world. There could have been more in - depth exploration of the different nations and their internal politics. Also, the power system of the Geass might have been more intricate, with additional rules or limitations that got simplified in the final version. It's possible that certain characters were meant to have different roles in the grand scheme of things, like some secondary characters being more prominent in the original concept.
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.