If it was a crossover with 'Bleach', Ichigo would be a great choice. His role as a Soul Reaper with his own set of powers could mix well with the world of 'Code Geass'. He might be resistant to Lelouch's Geass due to his strong will, and they could either end up as allies or opponents depending on the situation in the fanfic.
Lelouch would be really interesting. His complex personality and his ability to manipulate with Geass would make for some great interactions with Naruto. He could try to use Geass on Naruto, but Naruto's strong will might resist it, leading to a power struggle of sorts.
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.
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.
Some fanfics with character watch focus on the minor characters in 'Code Geass'. For instance, they might look at the character of Rivalz. They could show his reactions to the events happening around Lelouch and how he is affected by the bigger picture of the war and power struggles. This gives a fresh look at the series from an often - overlooked character's point of view.
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.
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.
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.
I think Ezio Auditore would be a great addition. He's a beloved character from the Assassin's Creed series. His skills and charisma would make for an interesting interaction with Kassandra or Alexios. They could learn from each other's experiences and combat styles. Another interesting character could be Lara Croft. Her knowledge of archaeology and survival skills would blend well with the world of Assassin's Creed Odyssey. She could join forces with the assassin to explore ancient tombs and uncover hidden secrets.
The Black Prince's character could develop through facing various moral dilemmas. For example, he might be forced to choose between loyalty to his family and doing what is right for the greater good. This would make him question his values and beliefs, leading to growth.
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.