One aspect that could be explored in such fanfictions is the idea of dependence. Light depends on Ryuk for the use of the Death Note, at least in terms of understanding its full capabilities. Ryuk, on the other hand, depends on Light for entertainment. This mutual need could be the basis for a deeper, more emotional connection in the context of yaoi fanfiction, where feelings of love or at least a strong bond could develop in unexpected ways.
In Death Note fanfiction, Ryuk and Rem have a rather complex relationship. Ryuk is more of an indifferent observer of the human world, while Rem has a deep emotional investment in Misa. Their interactions can be really interesting. Sometimes they might cross paths in the Shinigami realm or in relation to the actions of their respective human owners. Ryuk might find Rem's loyalty to Misa strange given his own non -chalant attitude towards Light. It's these differences that can create some really engaging scenarios in fanfiction.
Isolation is also a common theme. Light is isolated in his grand plan to create a new world, and Ryuk is isolated in his existence as a Shinigami. Their relationship in fanfiction can be a way to break that isolation for both of them. It can be a connection that neither expected but that becomes essential to their characters' development.
In 'Death Note Ryuk x Rem Fanfiction', a frequent theme is the contrast between their loyalties. Ryuk doesn't really have a strong loyalty to Light, he's just entertained by him. But Rem is extremely loyal to Misa. This contrast often leads to interesting scenarios where they either clash or come to some sort of understanding. Also, the theme of their different views on the power of the Death Note is common. Ryuk sees it as a source of amusement, while Rem is more cautious about its use due to her love for Misa.
One plot idea could be Ryuk trying to save Light from the consequences of his actions. Since Ryuk is an otherworldly being, he might have some knowledge or power that could potentially help Light avoid getting caught. For example, he could use his Shinigami powers in a way that no one expects to mislead the investigators on Light's trail.
You could also try FanFiction.net. Although it has a wide range of content, you can use the search function to find 'Death Note' fanfiction involving Ryuk and Light. Then, carefully read the summaries and reviews to ensure it's not lemon - themed. Some fanfics might focus on their unique relationship in a more platonic or plot - driven way, like exploring how Ryuk's presence affects Light's actions in the story from a different perspective.
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.