One idea could be to explore the bittersweet relationship between the main characters in a different time period. Maybe they meet in a modern-day setting but still have the same complex emotions as in the original Twilight. Another idea is to focus on the 'bittersweet symphony' aspect more literally, like having music play a crucial role in their relationship, perhaps a song that represents their love and the difficulties they face.
First, re - watch or re - read Twilight to really get a feel for the characters and the world. Then, think about what makes a relationship bittersweet. It could be unrequited love, or love that has obstacles. For the 'bittersweet symphony' part, find a song that gives you that feeling and use it as inspiration. Start with a simple scene, like the characters meeting in an unexpected place and feeling that mix of emotions.
I'm not sure specifically as I haven't read it. But generally, a graphic novel often tells a story through a combination of illustrations and text. It might be about a dark or dramatic theme considering the 'Symphony of Death' in the title, perhaps involving stories related to mortality, the afterlife, or some sort of epic struggle against death.
Overall, the Death Note Christmas comic could be a festive spin-off that maintains the dark and mysterious tone of the original series while adding holiday elements. It might explore new aspects of the characters' interactions during the Christmas season.
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.
I don't know the main characters in 'Symphony of Death Graphic Novel' as I haven't read it. But they could be characters like a reaper - like figure if it has a more traditional take on death, or perhaps a group of people trying to avoid an impending doom related to death.