One idea could be a crossover with 'Attack on Titan'. The mechs in Code Geass could be used in the fight against the Titans. Another idea is a crossover with 'Fate/Stay Night'. The mages and their magic in Fate could interact with the Geass powers in Code Geass. A third idea might be a crossover with 'Fullmetal Alchemist'. The concept of equivalent exchange could be explored in relation to the Geass power's consequences.
One idea could be a crossover with 'Fullmetal Alchemist'. Picture Lelouch using his Geass to manipulate the Homunculi in some way. Another could be a crossover with 'Attack on Titan', where the Geass power might play a role in dealing with the Titans. Maybe Lelouch could use his Geass to control some of the Titans and turn the tide of battle.
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 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.
One idea could be that Subaru somehow gets the Geass power. He could use it to try and save Emilia in a more decisive way, but he also has to deal with the moral implications of the Geass, just like Lelouch did. This would create a really interesting dynamic as Subaru is already a complex character dealing with his own set of challenges.
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 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.
A crossover might involve characters from different universes coming into the Code Geass world. Say, a character with superpowers from a superhero - themed anime enters the story. This could create a lot of chaos as Lelouch tries to either use this new element to his advantage in his fight against Britannia or has to deal with the unforeseen consequences of this new presence.
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 a crossover where characters from Harry Potter use Geass powers. For example, Harry could gain the Geass power to make Voldemort tell the truth about his Horcruxes. Another idea is to have Code Geass characters attend Hogwarts and learn magic. Lelouch could use his intelligence along with magic to outwit the Death Eaters. Maybe there could be a magical Geass, like a Geass that can control magical creatures in the Harry Potter universe.