One idea could be an OC who has the power to manipulate time in a limited way. They could use this power to try and prevent certain tragedies in the Code Geass world, like the fall of certain regions. Another plot could involve an OC who is a master strategist from a different universe and has to adapt their skills to the complex politics of Code Geass. A third idea might be an OC who can communicate with machines, which could lead to some really unique interactions with the mechs in the series.
A great plot for a Code Geass SI OC fanfiction could be an OC who has the power of empathy. They can feel the emotions of others deeply and use this to manipulate people in a more subtle way than Geass. This OC could start as a nobody but gradually rise in power by understanding the desires and fears of those around them. Another could be an OC who is from a parallel world where technology and magic co - exist. Bringing this knowledge into the Code Geass universe, they try to find a way to combine the two elements to create a new form of power. And an OC with the ability to teleport short distances could have some really exciting chase scenes and strategic maneuvers within the story.
One idea could be an OC who is a commoner but has a unique power that gets noticed by Lelouch. For example, the ability to manipulate sound waves which could be used for both stealth and attack. Another could be an OC from a foreign nation who has different moral and cultural values than the Britannians and Japanese, creating interesting conflicts. Or an OC who is a brilliant scientist working on a secret project that could change the balance of power in the Code Geass world.
A possible plot is the OC having a Geass that allows him to understand and communicate with all forms of life. This would make him very interesting to the different factions in Code Geass. The Britannian royalty might want to use him for political gain, while the resistance groups see him as a potential ally. Along the way, he forms a harem of characters from both sides who are drawn to his unique ability and kind heart.
One interesting plot idea could be that the OC is a hidden rebel within the Britannian forces. They initially cross paths with Cornelia during a battle, and the OC's unique abilities catch Cornelia's attention. As the story progresses, they start to understand each other's motives better, despite being on opposite sides. Maybe the OC has a secret power related to mind - reading which helps them anticipate Cornelia's moves, and this leads to a series of tense encounters that gradually turn into a strange sort of respect and eventually something more.
One idea could be an OC who has a special power related to time manipulation. They could use this power to try and change the course of events in the Code Geass universe. For example, they might attempt to prevent certain battles or alliances from forming. Another plot idea could be an OC who is from a completely different world and gets caught up in the conflict of Akito the Exiled. They could bring in new technologies or ways of thinking that disrupt the status quo.
An OC with a unique Geass power, like the ability to manipulate time on a small scale. For example, they could slow down time for a few seconds to gain an advantage in combat or to escape dangerous situations. This would add a new dynamic to the battles and strategies in the Code Geass universe.
The OC could be a time - traveler who lands in the Code Geass world and meets the prince. As she tries to find her way back home, she accidentally creates a harem situation. There could be a knight who is sworn to protect the prince and starts to have feelings for the OC because of her strange knowledge from the future, and a foreign princess who sees the OC as a way to form an alliance with the prince.
One idea could be the gamer having the power to manipulate the Code Geass world like a game. For example, they could level up their Geass power, unlocking new abilities. Another plot could involve the gamer character teaming up with Lelouch to outwit the Britannian forces using game - like strategies. Maybe they can see hidden stats of characters and use that knowledge to their advantage.
A really interesting plot could be a Code Geass character being turned into a vampire. This would completely change their dynamics. For example, if Lelouch became a vampire, he would have to deal with his new vampiric nature while still trying to carry out his plans for revolution. It could add a whole new layer of conflict and drama to the story.
One idea could be exploring an alternate timeline where Lelouch makes different choices in key moments. For example, what if he didn't use his Geass power on Euphemia? Another could be a story that focuses on the lives of the minor characters during the events of 'March of Time' and how they are affected by the main plot. Maybe a story about a common soldier's perspective in the Britannian army during all the chaos.
Another idea is that a character from the Naruto universe is transported to the Code Geass world and gains the Geass power. He then uses his ninja skills along with the Geass to build a harem. He might use his Geass to make female characters fall in love with him, but then has to deal with the moral implications of it, all while trying to survive in this new and dangerous world.