Yes, there are some interesting ones. You can start by looking on fanfiction.net. There are a variety of stories that explore the relationship between Sayu and L in different ways, from angsty dramas to more light - hearted takes on their interactions.
The unique aspect is that it combines two very different characters. Sayu is a more innocent and normal girl in the Death Note world, while L is this super intelligent and eccentric detective. Their pairing creates a contrast that can lead to really interesting storylines.
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.
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.