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.
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 idea could be that Naruto somehow gets the Geass power. He might use it in his battles against the Akatsuki. For example, he could use it to make an Akatsuki member reveal their secrets or to stop them from using their jutsu for a short time.
One idea could be a crossover where Naruto characters end up in the Code Geass world and have to deal with the political intrigue there. For example, Naruto might try to use his talk - no - jutsu on the Britannian royalty.
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.
Maybe a harem fanfic where Lelouch's Geass power has an unexpected side effect on the girls around him. Instead of just making them obey a single command, it creates a strong emotional bond that leads to the harem situation. The story could then explore how Lelouch deals with these new relationships while still trying to achieve his main goals 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.
Perhaps Lelouch from Code Geass ends up in the Naruto world. He could use his intelligence and strategic skills to help the shinobi against a common enemy. He might form an alliance with Naruto, and together they come up with plans to take on powerful foes like Akatsuki. Lelouch could also use his Geass to deceive the enemies and gain an upper hand in battles.
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.