Perhaps a plot twist where the Geass power is revealed to be related to the Quincy powers in Bleach. This could mean that there is a deeper connection between the two universes. If Lelouch was part Quincy all along, it would change his character arc. He could then have access to Quincy abilities like Blut Vene and Blut Arterie, which would give him an edge in combat against both Britannian forces and any potential threats from the Bleach universe.
One interesting plot twist could be that the Death Eaters in the Harry Potter world form an alliance with the Britannian Empire from Code Geass. This would create a powerful and evil force that the protagonists from both worlds would have to fight against. Another could be that a character like Lelouch, with his Geass power, gets transported to Hogwarts and uses his wits and power to outmaneuver Voldemort. Maybe he could even use his Geass on some of Voldemort's followers to turn them against him.
One possible plot twist is that the hogyoku from Bleach somehow ends up in the hands of a character in Highschool DxD. This could lead to a character attempting to use it for their own gain, perhaps to gain ultimate power over both the devils and the angels. Another interesting plot twist is that a character from Highschool DxD gets reincarnated as a Soul Reaper in Bleach. They would have to learn the ways of the Soul Reapers while still retaining some of their previous abilities and memories. This could create some really interesting character development. A third plot twist could be that the balance between the three factions in Highschool DxD (devils, angels, and fallen angels) is disrupted by the arrival of Hollows from Bleach. The factions would have to figure out how to deal with this new threat, and it could lead to some unexpected alliances.
One interesting plot twist could be a character from Bleach accidentally getting sent to Yokai Academy. Maybe Ichigo ends up there and is mistaken for a new yokai student. His reiatsu could be misinterpreted as some kind of strange yokai power, leading to all sorts of chaos as he tries to figure out how to get back home while also dealing with the unique challenges of the academy.
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.
One idea could be that Naruto somehow gets the Geass power. He might use it in his battles against the Akatsuki. For example, he could use it to make an Akatsuki member reveal their secrets or to stop them from using their jutsu for a short time.
The relationship between C.C. and Rukia would be quite complex. C.C. has a very mysterious and detached personality, while Rukia is more duty - bound and has a strong sense of camaraderie. They could start off as rivals, both being strong female characters with unique powers. However, as they get to know each other's pasts and motives, they might develop a grudging respect. C.C. might be intrigued by Rukia's connection to the Soul Society, and Rukia could be curious about C.C.'s immortality and her role in Lelouch's life.
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.
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 interesting plot twist could be the sharing of powers. For example, Naruto's chakra abilities might combine with a Bleach character's spiritual power in an unexpected way. Maybe Naruto could gain the ability to use a zanpakuto-like weapon with his chakra, which would be a huge change in the battle dynamics. Another plot twist could be a character from Bleach getting access to Naruto's sage mode knowledge, leading to a new form that combines the two universes' powers. And of course, there could be a plot twist where the villains from both universes team up, forcing the heroes to put aside their differences and come up with a new strategy to fight against them.
One idea could be an OC who is a commoner but has a unique power that gets noticed by Lelouch. For example, the ability to manipulate sound waves which could be used for both stealth and attack. Another could be an OC from a foreign nation who has different moral and cultural values than the Britannians and Japanese, creating interesting conflicts. Or an OC who is a brilliant scientist working on a secret project that could change the balance of power in the Code Geass world.