Perhaps a fanfic could explore a scenario where Rai has to pretend to be a normal human for some reason, and Frankenstein has to help him blend in. This would be full of comical moments as Rai tries to understand human customs and behaviors, all while Frankenstein is there to keep him out of trouble and protect his true identity.
One idea could be that Rai, with his unique powers, somehow stumbles upon the re - animated creation of Frankenstein. They initially see each other as threats but gradually come to understand that they both are outcasts in a way. Rai could use his powers to help Frankenstein's creature control his more violent tendencies. Another plot could involve them teaming up against a common enemy, like a mad scientist who wants to use both of them for his own evil experiments.
You can try fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfics, and you can simply search for 'Noblesse Rai x Frankenstein' there. Many talented writers share their works on it.
Another plot could involve the creation of a female monster. In the original, Victor Frankenstein was hesitant to create a female. A fanfic could explore what would happen if he did. Maybe the two monsters would have a complex relationship, and together they would try to find a place in the world that would accept them.
One common theme is the exploration of their relationship. Since they are both important characters in Noblesse, fanfics often delve into how they interact with each other, their feelings towards one another, whether it's a platonic bond or something more romantic.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfics on various topics, including Noblesse. Just search for 'Noblesse Rai x Raskreia' and you'll likely find some interesting reads.
One idea could be that the OC is a healer who stumbles upon Frankenstein's monster in the woods. The OC decides to help the monster, despite the danger, and they form an unlikely bond. This could lead to a story of redemption for the monster and a journey of self - discovery for the OC.
The plot changes would be interesting. Naruto's presence might make the Noblesse characters question their own values. His journey to be recognized and respected in his own world could translate into a similar arc in the Noblesse world. He could also introduce new forms of combat and training. For example, his ability to learn new jutsus quickly could inspire some of the Noblesse characters to seek new ways of increasing their power. And his sense of loyalty and friendship would surely shake up the relationships in the Noblesse fanfic.
Perhaps in the fanfiction, Rai is framed for a crime he didn't commit. Kallen believes in his innocence and decides to help him clear his name. As they dig deeper into the conspiracy, they uncover a much larger plot that threatens the entire Code Geass world. Their investigation brings them closer together as they work together to expose the real culprits and save their world.
One idea could be that Orochimaru tries to create a Frankenstein - like being using ninja techniques and DNA from various powerful shinobi. He might mix the strength of the Uchiha's Sharingan with the brute force of the Akimichi clan. Another could be Naruto accidentally stumbling upon a hidden laboratory where a mad scientist - type character is creating a Frankenstein creature. The creature could initially be hostile but then form an unlikely bond with Naruto as he shows it kindness.
One interesting 'Code Geass Rai Fanfic' could be about Rai getting involved in a political intrigue within the Code Geass universe. Maybe he teams up with Lelouch to outwit the Britannian forces. Another could be a story where Rai discovers a new Geass power that changes the power dynamics in the story.