Maybe the Black Prince discovers a new form of Geass power that no one else has ever seen. He then has to figure out how to control it while also hiding it from those who would seek to use it for their own evil purposes. This could involve a journey of self - discovery as he battles his inner demons as well.
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 story where Cornelia and Guilford have to go undercover in an enemy territory. They would have to rely on each other more than ever, facing challenges that test their loyalty and their relationship. Maybe they discover a secret plot that could change the course of the war in Code Geass.
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.
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 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.
A fanfiction could explore what would happen if the Black Knights discovered a new Geass power within their ranks. This could lead to internal power struggles as well as new strategies in their fight against the Britannian Empire. They might have to decide how to use this new power, whether for good or for their own gain, and how it would affect their relationships with each other.
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.
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 exploring an alternate universe where Shinkiro has different abilities or a different origin story. For example, what if Shinkiro was created by a different group with a different purpose in mind.
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.