Well, one of the main plot points is the emergence of new Death Notes and new users. There are different individuals who get their hands on the Death Notes and start using them in various ways, which leads to a complex web of power struggles and moral dilemmas.
The accelerated world novels can be arranged in the following order:
[1]"World Acceleration"
2. A Song of Ice and Fire
3 "Battle Through the Heavens"(Heavenly Silkworm Potato)
4. The Three-Body Problem (Liu Cixin)
5 "Infinite Terror"(Chen Dong)
6 "Full Time Expert"(Butterfly Blue)
[Lord Snow Eagle](Ethereal Journey)
8 "Choosing the Heavens"(Tricky)
9 Douluo Continent (Tang Family's Third Young Master)
10 "Sword Comes"(Beacon Fires Play with the Vassals)
It should be noted that these novels are all of different types. The order can be adjusted according to your preferences and reading needs.
The shinobi world's reaction would be complex. At first, there would be disbelief. Many would think it's some kind of new genjutsu or a hoax. But once they realize it's real, the more superstitious ninja would see it as a curse from the gods. The Hidden Sand Village might see it as a potential weapon against their enemies, while the Hidden Mist Village might be more cautious due to their history of bloodshed. The shinobi academies would start teaching about the Death Note as a new and dangerous threat. There would also be a black market for Death Note information, with some trying to sell false ways to protect against it.
To speed up the story of the world, you might consider increasing the pace of action and dialogue. Cut down on long descriptive passages and get straight to the core of the story. Another option is to introduce time jumps to cover larger periods quickly.
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.