Well, a very frequent theme is the exploration of power dynamics. Given Illumi's powerful and somewhat intimidating nature, the fanfictions often look at how the OC fits into that power structure. It could be that the OC is weak at first and has to grow to be on par with Illumi, or the OC might have a unique power that changes the balance. Also, love and redemption are common themes. The OC might bring out a sense of redemption in Illumi, or they might fall in love in a rather complex and interesting way considering Illumi's character.
Family drama is another big theme. The Zoldyck family has a complex family structure and a lot of internal politics. OCs in these fanfictions usually get caught up in this web of family drama, like dealing with rivalries between family members or trying to gain the approval of the family elders. A good example is a fanfiction where an OC has to navigate the feud between two Zoldyck siblings.
One popular illumi zoldyck x oc fanfiction is 'A Hunter's Heart'. It has an interesting plot where the OC character challenges Illumi's cold - hearted nature. Another great one could be 'Illumi's Unforeseen Bond'. These fanfictions often explore different aspects of Illumi's character in relation to the original creation by the author.
One common theme is adventure. Since Killua is from the Hunter x Hunter universe which is full of dangerous quests and exploration, many 'killua zoldyck x oc fanfiction' stories involve the OC joining him on these adventures. Another theme is friendship evolving into love. The OC and Killua start as friends or allies and then their relationship deepens over time.
One common theme is friendship. The SI OC often forms a deep bond with Killua, and they go through various adventures together, relying on each other. Another theme could be self - discovery. As the OC is in a new world with Killua, they learn more about themselves.
Friendship or brotherhood is also a big theme. Despite their differences, many fanfictions show how they support each other in their own ways, like Killua relying on Illumi's skills in certain dangerous situations and Illumi having a soft spot for Killua deep down.
Love and loyalty are themes that are often explored as well. It's not always romantic love, but a deep, unwavering loyalty that might exist between them. Some fanfictions show how, despite their differences, they would still be there for each other in difficult situations, like when facing a common enemy or dealing with the challenges within their family.
Another good one is 'My Life as a Zoldyck OC'. It stands out because it delves deep into the Zoldyck family secrets from the perspective of the OC. The story is full of twists and turns, like when the OC discovers a hidden part of the Zoldyck mansion that no one else knew about.
One common theme is adventure. Since Killua is from the Hunter x Hunter world which is full of dangerous quests and exploration, many fanfictions involve the reader joining him on his adventures. For example, they might go on a journey to find a rare treasure or explore a mysterious and dangerous place.
One great 'oc Zoldyck fanfiction' could be one where the OC is a long - lost relative who tries to fit into the Zoldyck family's unique lifestyle. They might have to learn the family's assassin ways while also bringing in their own values.
You can try Archive of Our Own (AO3). It has a large collection of fanfictions, and you're likely to find many 'si oc zoldyck' ones there.