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.
Another plot could involve the sharing of powers between the two universes. Some ninjas could gain Geass - like abilities, while some Code Geass characters could learn ninja techniques. This would create a whole new dynamic of power and conflict. For instance, Sasuke could get a Geass power that allows him to see the true intentions of his enemies, which would help him in his quest for revenge and also in his decision - making process.
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 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 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.
One plot idea could be exploring Cornelia's relationship with her subordinates in a more in - depth way. Maybe she has to lead a risky mission and how she overcomes the challenges within the team. Another could be a story set in an alternate universe where Cornelia has to make different choices that change the fate of the Britannian Empire.
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.
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.
One idea could be exploring Suzaku's internal struggle in a different light. Maybe he discovers a new power within himself that makes him question his loyalty to Britannia even more. For example, he could have a dream where he sees an alternate future of Japan if he fully embraced the Japanese side from the start.
One idea could be a story where Nunnaly discovers her hidden power in the Code Geass world. Maybe she has a special ability that was dormant all this time and it starts to surface when she is in a life - threatening situation. This could change the power dynamics in the story and her relationship with other characters.
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.
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.