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.
Characters like Near might analyze the fanfiction. He'd probably look at the logic behind the plotlines in the fan - made stories and compare them to the real events in the Death Note world. If the fanfiction has a really convoluted and illogical plot, he might dismiss it as being not worthy of his attention. But if it has an interesting take on the characters' motives, he might give it more thought.
Some characters might be shocked at the alternate interpretations of their personalities and actions. For example, Light might be appalled if the fanfic portrays him in a very different light than he sees himself. He's so self - assured in the original series that seeing a different version of his motives could be quite a jolt.
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.
Oh, for sure! Death Note is considered a masterpiece in the manga world. Its unique concept and the way it unfolds make it a must-read. The characters' development and the moral dilemmas presented make it stand out from the crowd.