Well, one common theme is probably the struggle for power. Rido Kuran is a very powerful pureblood, and Zero Kiryuu has his own strength as a vampire hunter. Fanfics might play around with how they interact in terms of power. Another theme could be redemption. If they are in a relationship, perhaps one is trying to redeem the other. For example, Zero might try to redeem Rido from his more evil tendencies, or vice versa.
Fanfictions often show Zero's character development in relation to Rido by making him question his hatred for vampires. Since Rido is a powerful pureblood, Zero's interactions with him can force him to reevaluate his views. For Rido, his development might be shown as a change in his power - hungry ways due to his relationship with Zero.
In a Zero Kiryu x Rido Kuran fanfic regarding unlikely encounters, the initial reaction between them would be very interesting. Zero, with his hatred for vampires (despite being one), would likely be hostile towards Rido. Rido, seeing Zero as an inferior at first, might underestimate him. However, as they interact more, they could find that they have more in common than they thought, like their determination in achieving their goals, which could lead to a change in their relationship dynamic.
One interesting aspect of a Zero Kiryu x Rido Kuran fanfic could be the character development. Zero might start off as completely opposed to Rido, but as the story progresses, he could come to understand some of Rido's motives, not necessarily agreeing with them, but seeing the deeper reasons behind his actions. Rido, on the other hand, could see in Zero a worthy adversary or even a potential tool for his own plans. This could lead to a complex web of relationships and plot twists in the fanfic.
One common theme is the struggle between love and duty. Rido Kuran often has responsibilities within his world, and his love story might be in conflict with those. Another theme is the power dynamics. Since he may have certain powers, how those interact with his love relationship can be explored. For example, does his power make the relationship more difficult or does it protect his loved one?
One common theme is the exploration of their underlying feelings. Despite their apparent hatred in the original work, fanfictions often reveal hidden affection. Another theme is redemption, where they help each other overcome their personal demons.
The theme of redemption can also be seen. Zero, with his inner struggle against his vampire side, might find redemption or acceptance through his interactions with Kaname. And Kaname, who has his own secrets and burdens, could also experience a form of redemption in relation to Zero. In many fanfictions, their relationship is a vehicle for exploring these deeper character arcs.
There are some 'rido kuran x oc fanfiction' stories that focus on the OC being from a different clan. This creates an interesting conflict as Rido Kuran, with his own motives and the OC has to navigate their relationship while also dealing with the politics between clans. It often leads to a lot of drama and unexpected plot twists.
The theme of power and hierarchy within the vampire world is also quite common. Kaname being an important figure in that world, fanfictions might show his dealing with political intrigue, power struggles among different vampire clans. And his relationship with the Pureblood vampires and how he navigates that complex social structure is often a part of these stories. For example, some fanfictions might focus on how he tries to reform the old ways of the vampire hierarchy to protect both vampires and humans.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, and you're likely to find some great 'rido kuran x oc fanfiction' there.
Another theme is the power of friendship turning into love. They start off as comrades or friends perhaps, and then the story shows how their relationship deepens into something more romantic. For example, in some fanfics, they go through adventures together which bring them closer and closer until they realize their true feelings for each other.