I really like 'Geralt's Claim'. In this fanfic, Geralt's possessiveness is a bit more intense. It starts with a situation where Jaskier is almost kidnapped, and from then on, Geralt is constantly by his side. The story is filled with tender moments between them, like when Geralt gently tends to Jaskier's wounds after a small skirmish, all while his possessive side is simmering just beneath the surface.
The exploration of their powers is also a theme. Geralt's Witcher abilities and Yennefer's magic often interact in interesting ways in fanfiction. For example, they might combine their powers to defeat a powerful enemy. And of course, the theme of redemption is sometimes there. They both have made mistakes in their pasts and in fanfiction, they might help each other find redemption.
Friendship is a very common theme. Their loyalty and the way they support each other through various trials. For example, in many fanfictions, Geralt protects Dandelion from monsters while Dandelion uses his charm and words to get them out of tricky social situations.
Another theme is unrequited love or the slow - burn of a romantic relationship. Some fanfictions focus on the idea that Jaskier has deeper feelings for Geralt that he may not be fully aware of himself, and how that develops over time.
First, you need to understand Geralt's character well. Read the original books or watch the series. Know his values, his fighting style, and his typical reactions. Then, create an OC that can interact meaningfully with him. For example, if your OC is a healer, they could have different views on how to deal with the monsters Geralt hunts.
One major theme is mutual understanding. Jaskier and Geralt come from very different backgrounds, and fanfictions like to show how they come to understand each other better. Also, the theme of sacrifice is present in some. For example, Jaskier might sacrifice his own comfort or safety for Geralt's sake, showing the depth of his care. And of course, there's the theme of love, whether it's platonic or romantic love, which is explored in different ways in these fanfictions.
One common theme is adventure. Geralt is often depicted going on quests with the OC, facing various monsters and challenges. This allows for exciting action scenes and exploration of the Witcher world.
One interesting Geralt of Rivia OC fanfiction could be one where the OC is a powerful mage who teams up with Geralt to fight a new and mysterious dark force. They might have a complex relationship, with the mage initially distrusting Geralt's methods but gradually coming to respect him.
I would recommend 'A Song in the Wind'. It delves deep into how Geralt and Jaskier support each other. Jaskier's music is beautifully intertwined with the story, and Geralt's protective side towards Jaskier is shown in a very endearing way. It gives a good overall view of their dynamic.