There was no official news about the sequel to The Final Note. However, according to the order of the original novel, the first and second parts of Grave Robber's Chronicles 8 should follow after The Ultimate Chronicles. This novel mainly talked about Wu Xie wearing the human skin mask of Wu Sansheng, reorganizing his third uncle's Changsha industry, and starting a new adventure with Pan Zi, Xiao Hua, and others. Although there is no specific information about the sequels, readers can look forward to continuing to follow the story development of Wu Xie and other characters.
I'm not sure yet as it's a new story. But it could be new characters who discover the Death Note, or perhaps some descendants of the original characters like Light or L.
Perhaps in the 'new death note story', the Shinigami who are associated with the Death Note have changed their ways. They could be interfering more directly in the human world, either to protect the Death Note or to stop its misuse. This would add a whole new layer of conflict as humans would have to deal not only with the power of the Death Note but also with the meddling Shinigami.
It could be a young detective who is trying to solve the mystery of the Death Note's reappearance. He might be really smart and determined to stop whoever is using the Death Note for evil purposes.
There was no official sequel to the death train novel. However, some novelists would create sequels or side stories based on their own creativity and ideas. However, these works might not necessarily be the same as the original novel. There might be some modifications or cuts, and they might not be created by the original author. If you want to read the death train novel's sequels, it's recommended to read the original first or wait for the author to release new updates.
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.