In a crossover, the two different power systems would create a really interesting dynamic. The Death Note's power to kill anyone whose name is written in it, and the Geass which can control others' actions. If Light and Lelouch were to cross paths, there would be a battle of wits as they both are highly intelligent characters. Light would be cautious of Lelouch's Geass, and Lelouch would be wary of the Death Note's invisible and deadly power.
It's hard to say who would be the most powerful. Light has the power of the Death Note which can kill anyone directly. However, Lelouch has the Geass which can be used in very strategic ways to manipulate people. In a direct confrontation, if Light knew Lelouch's real name, he could write it in the Death Note. But if Lelouch could use his Geass on Light before that, he could make Light do things against his will.
In such a fanfiction, the story could start with Naruto being summoned to the Code Geass world by some strange magic or technology. He'd be confused at first, but then quickly adapt. The Akatsuki could also be brought into the story as a sort of anti - Britannia group. And the Geass users would be both intrigued and cautious of Naruto's power. There could be epic battles between the ninja and the mecha, and lots of drama as the two different cultures collide.
It could be a combined force of Orochimaru and some of the power - hungry nobles from Code Geass. Orochimaru is always after power and knowledge, and the nobles in Code Geass are often corrupt. Together, they could create a really formidable enemy for the heroes.
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.
Light Yagami from Death Note could be a great character for a crossover. Both he and Lelouch are highly intelligent and are willing to do whatever it takes to achieve their goals. A meeting between them would be a battle of wits, with each trying to outmaneuver the other, either to use the other's power for their own benefit or to eliminate a potential threat.
The plot might involve Sakura being in a moral dilemma. Since she's a healer in the Naruto world, using the Death Note which is a tool for killing would go against her nature. So, it could be about how she struggles with this new power that she has and tries to find a way to use it without sacrificing her values.
One possible plot could be that Light Yagami discovers the existence of demons from the Inuyasha world. He might see them as a new type of 'evil' to be eradicated. He could start using the Death Note on demons, which would draw the attention of Inuyasha and his group. Inuyasha would initially be confused about these strange deaths and then realize that a human is causing them. This would lead to a conflict as Inuyasha believes in slaying demons in a more 'honorable' way and not through this strange notebook's power.
One great Code Geass crossover fanfiction is 'Code Geass and Naruto: A World in Chaos'. It combines the political intrigue of Code Geass with the ninja world of Naruto. The story has Lelouch using his Geass in the Naruto universe, which creates a whole new set of challenges and alliances. Another is 'Code Geass meets Attack on Titan'. In this one, the Titans pose a new threat to the Code Geass world and Lelouch has to figure out how to deal with them. It's interesting to see how the different power dynamics play out.
Madara Uchiha could be a great main villain in such a crossover. His already powerful ninja abilities combined with the potential of getting a Geass power would make him extremely dangerous. He could use Geass to control other ninjas and make them fight for his cause, like creating an army of mind - controlled shinobi to take over the world.
A 'Naruto Death Note Crossover Fanfic' could be really interesting. Maybe Naruto somehow gets his hands on the Death Note. He might initially be confused by it, but his strong sense of justice could lead him to use it in a way to protect the Hidden Leaf Village from dangerous enemies. For example, he could use it to take out rogue ninjas who are planning mass destruction without having to engage in a long and dangerous battle.