A cool concept could be that the Geass power is somehow linked to the Mother Boxes in the Justice League lore. C.C. discovers this link and tries to convince the League to help her and Lelouch use this connection to stop an even greater threat that is about to destroy both their worlds. This would involve a lot of cooperation and understanding between the two very different groups.
Maybe in the fanfic, the Justice League gets transported to the Code Geass universe. Batman could start analyzing the political and power structures there, while Wonder Woman is intrigued by the concept of Geass. They could team up with the Black Knights, with Flash helping with quick rescues during battles. And Aquaman might find some similarities between his underwater kingdom and the various factions in the Code Geass world.
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.
One idea could be exploring the backstories of the lesser - known members in more detail. For example, Martian Manhunter's life on Mars before coming to Earth.
For a 'Code Geass Lelouch Ressurrection' fanfic, a great concept would be to set it in an alternate universe where the resurrection has some unforeseen consequences. For instance, the balance of power in the world has completely shifted. Lelouch has to navigate this new world order while also dealing with his own internal struggles about his identity and purpose.
You could also write a fanfic focusing on the relationships within the royal family. There could be a story about a young royal trying to gain the approval of their strict and traditional parents. Or perhaps a love story between two members of the royal family who are not supposed to be together due to family politics and power struggles.
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 popular idea could be alternate universes where the characters have different roles or abilities. For example, a fanfic where Lelouch has to lead a rebellion without his Geass power. Another idea might be character relationship exploration, like a deeper look into the friendship between Suzaku and Lelouch before their paths diverged so much. And there could also be fanfics that focus on the backstories of minor characters in the Code Geass world, giving them more depth and significance.
You could base the plot on a time - traveling element. Suppose a villain from the future travels back in time to the Code Geass and Justice League worlds to change history in their favor. The Justice League and Lelouch's group must figure out the villain's plan and stop them. Along the way, there could be misunderstandings between the two groups due to their different cultures and methods. But as they learn more about each other, they form a stronger alliance to save both of their timelines.