If Light was younger, he might be less experienced and make more impulsive decisions, which could lead to a very different outcome in the story.
If Light was younger, he could have a harder time hiding his actions and might get caught sooner. His lack of life experience could also mean he underestimates his opponents, causing major shifts in the story's progression.
His view on life and death would be challenged. In Naruto's world, he fights to protect life, but the Death Note's power to kill so easily might make him more cautious and serious about the value of life.
Ryuk would probably just be amused. He is a Shinigami who has seen a lot in his existence. Fanfiction is just another strange human creation to him. He might chuckle at some of the more outlandish ideas in the fanfictions, like a character having superpowers beyond the Death Note.
Naruto's personality could become more brooding. Having the power of the Death Note might make him more aware of the darker side of power. He might not be as open and friendly as he usually is, since he has this secret and rather sinister tool at his disposal.
Naruto would likely face an internal conflict. His normal way of dealing with problems is through hard work, training, and believing in his friends. But the Death Note offers an easy way out, a way to kill without a fight. This would make him question his values. For instance, he might wonder if it's okay to use it on someone like Pain, who caused so much destruction in the village. On one hand, it could save many lives in the future, but on the other hand, it goes against his basic ninja beliefs.
The theme of sacrifice also exists. L is aware of the risks he takes in dealing with the power of the Death Note and is willing to sacrifice his own safety for the greater good. And there's the exploration of human nature when faced with such a powerful and dangerous tool as the Death Note.
Misa would likely be excited. She is a big fan of Light and the whole 'Kira' concept. In fanfiction, if there are scenarios where she has more power or a closer relationship with Light, she would be all for it. However, she might also get jealous if there are other female characters getting too close to Light in the fanfictions.
She might become more confident, or overconfident. Having the Death Note gives her a huge advantage over her enemies. She could start to rely too much on this power and think she can solve all problems with it. This could lead to her making mistakes and alienating her friends.
Naruto would first be extremely confused by the rules of the Death Note. His understanding of power and its consequences is rooted in his ninja training and his own moral compass. The Death Note's rules, which seem so simple yet so deadly, would be completely foreign to him. He would question why anyone would create such a thing and would be very hesitant to even consider using it, as it goes against everything he stands for in his world of ninjas, friendship, and protecting the innocent.
It focuses on the story of L's early days and how he solves cases. In this novel, we see L's unique detective methods and his battle of wits against various criminals.
Yes, Death Note is originally a light novel. It gained immense popularity and was later adapted into various forms like manga and anime.