Perhaps in the fanfic, Code Geass technology somehow finds its way into George Washington's time. He could use it to outwit the British forces. For example, he might use a Lancelot - like mech to turn the tide of a battle. The story could also explore how his values would interact with the often morally gray world of Code Geass.
He could be a rebel leader like Lelouch. Just as Lelouch fought against the Britannian Empire, Washington could fight against an oppressive regime in the Code Geass world. His leadership skills and strategic mind would be valuable assets.
One possible plot could be Lelouch finding himself in Washington's America and using his Geass to influence the political situation. He might encounter key historical figures and use his wits and power to change the course of events in a way that reflects the complex themes of the original 'Code Geass' series.
Suzaku could be another main character. His conflicted loyalties in the original series could be translated into this new setting. He might be on the side of the British at first, but then have a change of heart after seeing the injustices they are committing in America. His piloting skills could also come in handy, perhaps in battles against the British forces that have advanced mecha technology, similar to the Knightmare Frames in the original 'Code Geass'.
One possible interesting plot could be a crossover where Mario, from the Mario universe, somehow gets transported to the Code Geass world. He might use his unique abilities like jumping and power - ups to help the resistance in Code Geass against the Britannian rule. Diesel could be some new type of mecha or vehicle in this fanfic that is created by combining the technology of both worlds. For example, Mario could use his plumber skills to fix the Diesel mecha when it breaks down during a battle, adding an unexpected yet fun element to the story.
A fanfic could center around Cornelia's relationship with her sister, Euphemia. It could show their sisterly bond, their differences, and how they support or oppose each other in the complex world of Code Geass.
The plot could center around George Washington's relationships while he was leading the Continental Army. There could be a fictional female character who falls in love with him and supports him through the hardships of the war. This character might also be involved in some of the political and social events of the time, adding a romantic angle to the historical context.
Well, the plot could involve her being a secret power player in the political intrigue of the Code Geass world. Maybe she has her own Geass ability that she keeps hidden. She could be working behind the scenes to either support Schneizel's plans or have her own motives for the future of Britannia.
One possible plot idea could be that the OC Prince, who might be from a different region or have a unique background in the Code Geass universe, meets Kallen during a battle or a political intrigue. Their initial encounter could be filled with suspicion and hostility, but as they are forced to work together, they gradually develop feelings for each other. For example, they might have to team up to rescue a mutual friend or stop a new threat to Britannia and the Black Knights.
The plot could be Suzaku goes back in time and tries to save Euphemia. He believes that by changing that one event, a lot of the subsequent tragedies can be avoided. Lelouch, unaware of Suzaku's time - traveling at first, continues with his original plans. CC, who senses the change in the timeline, decides to find out what's going on and gets involved with both of them.
In this fanfic, a likely plot twist could be related to Lelouch's character. Maybe he has a hidden plan that even his closest allies don't know about. He could be manipulating events in a way that seems counter - intuitive at first but actually serves a greater long - term goal. Also, the Geass power might have some unforeseen evolution. It could start to affect the user in unexpected ways or have new limitations that force the characters to adapt their strategies.