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.
You can start by using the search function on TV Tropes. Just type in 'Code Geass fanfic' and it should bring up a list of related works. Look for ones with high ratings or a lot of reviews, as these are usually good indicators of quality.
One popular trope is the 'what - if' scenarios. For example, what if a different character got the Geass power instead of Lelouch? Another is the alternate universe (AU) trope, where the setting or the relationships in the story are completely different from the original Code Geass world. There are also many fanfics that focus on the relationships between characters, like a deeper exploration of the friendship or rivalry between Suzaku and Lelouch.
One popular trope is the 'fish out of water' situation. For example, when a character from Harry Potter is suddenly thrust into the world of Code Geass or vice versa. They have to learn new rules and ways of life. Another is the power - up trope. A character might gain new abilities by combining magic from Harry Potter and the Geass power. For example, a wizard could use a spell to enhance the effectiveness of their Geass.
One great trope is the 'Underdog Rising'. Your male OC could start as a nobody in the Code Geass world but gradually rise to power through his wits and determination. Another is the 'Redemption Arc'. Maybe he has made some mistakes in the past and is trying to make up for them. Also, the 'Mysterious Stranger' trope can work well. He arrives in the middle of the chaos in the Code Geass world with a hidden agenda that slowly unfolds.
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.