One interesting 'Death Note Reaction Fanfiction' could be about the characters' reactions when they first discover the Death Note. For example, Light's initial shock and then his quick realization of the power it holds. Another might focus on how L reacts to the existence of the Death Note and Light's actions. There could also be fanfictions that explore the reactions of secondary characters like Misa when she gets involved with the Death Note's power.
When writing a 'Death Note Reaction Fanfiction', it's important to stay true to the tone of the original series. The Death Note world is dark and full of moral ambiguity. So, when writing about the characters' reactions, keep that in mind. For instance, if you're writing about L's reaction to a new lead in the Kira case, show his meticulous thinking process. You can also add your own twists. Maybe the Death Note has a side effect that makes the user see hallucinations. How would the characters react to that? Plan out your story structure, with a clear beginning, middle, and end, and make sure the reactions of the characters drive the plot forward.
The ending of 'Death Note' is quite complex. Essentially, Light, the main character who had been using the Death Note in his self - perceived quest for justice, is finally defeated. His downfall shows that his extreme methods of creating a 'new world' through killing those he deemed as criminals were ultimately wrong. It's a cautionary tale about the abuse of power and the distortion of justice.
In fact, the manga of Death Note has its unique ending that sets it apart. It's a conclusion crafted specifically for the original comic format, with elements and developments not found in other versions.
At the end of 'Death Note', Light's character deviated far from his initial self. He began as a student with a strong sense of justice, but the Death Note's power made him believe he could be a god - like figure dictating who should live and die. Misa, who was initially in love with Light and a part of his plans, was left broken in the end. L had a consistent development of being a brilliant mind in the hunt for Kira. His successors, Near and Mello, also grew in their own ways in the fight against Light, with Near finally outsmarting him in the end.
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.
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.