A frequent theme is the exploration of what would happen if the power of the Death Note was introduced to the Naruto universe. It could disrupt the balance of power that exists in Naruto's world. Also, the theme of identity can be seen as Light Yagami has to adapt to a new world and its rules while still being himself.
In many fanfictions about Light Yagami and Misa, their relationship is often complex. Light initially uses Misa for his own ends in the 'Death Note' story. He manipulates her feelings because of her Shinigami eyes which are useful for his plan to create a new world. Misa, on the other hand, is blindly in love with Light. In fanfiction, this dynamic can be explored in different ways. Some might show Misa slowly realizing she's being used, while others may focus on a more 'romanticized' version of their relationship where Light actually develops some real feelings for Misa.
Sure. One interesting 'naruto light yagami fanfiction' could be where Light Yagami somehow gets transported to the Naruto world and tries to use the Death Note in a world full of ninjas. It makes for a really unique and exciting story.
One unique aspect is the clash of different moral systems. For example, when Light crosses into a universe like 'Star Trek' where the Federation has a very different view on life and justice. His use of the Death Note would be highly controversial. Another aspect is the exploration of his character in new settings. In a crossover with 'Fullmetal Alchemist', he would be exposed to a world full of alchemy and different power structures.
Maybe Misa. She's completely devoted to Light, but what if she discovers that Light has been using her all along? In a fit of rage and heartbreak, she could kidnap him. She has the means as she's also involved with the Death Note. She might want to make Light pay for his betrayal or force him to love her truly.
One common theme is the exploration of gender - based differences in handling power. Since Light is a very power - hungry character in the original, in fanfiction with a female Light, it might be about how she uses the Death Note differently because of her gender. For example, she could be more empathetic or more calculating in different ways than the male Light.
Some fanfictions focus on Light's relationships. For example, how his relationship with his family, especially his father, might have changed if things had gone differently. There could also be fanfictions that explore a romantic relationship for Light, which is quite different from his single - minded pursuit of his 'new world' in the original story.
One idea could be that Light Yagami discovers the wizarding world. He might see the use of magic as a way to enforce his idea of justice more effectively. For example, he could use spells to secretly observe people's actions and then use the Death Note on those he deems as criminals in the wizarding world. However, he would soon face challenges as the wizarding laws and ethics are different from what he's used to in his own world.
I'm not sure specifically which 'Kazuma Yagami' in a light novel as the name is not widely known in extremely popular mainstream light novels. It could be a character from a relatively niche or new light novel series.
A very prevalent theme in 'Light Yagami x Reader fanfiction' is the idea of trust. Given Light's secretive nature, the fanfictions show how the reader character tries to gain his trust or vice versa. Also, the theme of manipulation is common. Light is a master manipulator in the original series, and in the fanfictions, this aspect is often explored in relation to the reader. For example, in some stories, Light might try to use the reader for his own ends. Additionally, there's the theme of redemption in some fanfictions, where the reader tries to 'save' Light from his dark path.