I'm not sure if it's a highly popular genre specifically. It might have a niche following among those interested in medieval or royalty - themed stories.
I'm not entirely sure as I haven't experienced it directly. But generally, a visual novel often involves a story with characters and plotlines. Since it has 'kingsguard' in the title, it might be related to a group of guards in a king - like setting. And 'furry' could imply the presence of anthropomorphic animal characters.
One great 'oc kingsguard fanfiction' could be one where the OC has a unique backstory that ties into the world of the Kingsguard in an unexpected way. For example, an OC who was once a street urchin but through a series of events becomes a part of the Kingsguard and has to learn the ways of honor and loyalty quickly.
Since I haven't played or read it, I can't say for sure. But if it follows the typical pattern of visual novels, there are likely to be a variety of interesting characters. Given the 'kingsguard' and 'furry' aspects, there could be brave guards with unique animal - like features and personalities.
One interesting plotline could be a Kingsguard member having to choose between loyalty to a tyrannical king and protecting the innocent. For example, if the king orders unjust executions, the Kingsguard might struggle with their oath.
Ser Jaime Lannister is often a popular character. His complex character arc in the original series makes him ripe for further exploration in fanfiction. His internal struggle between honor and family loyalty can be delved into more deeply.
First, you need to develop your OC. Decide on their personality, background, and what makes them unique. For example, are they a skilled fighter with a tragic past? Then, think about how they fit into the world of the Kingsguard. Do they have different values compared to the traditional Kingsguard members?
One idea could be Jon Snow being the Kingsguard during a time of great political unrest in Westeros. He has to protect a young and inexperienced king from various assassination attempts by rival houses. Another plot could involve him having to choose between his loyalty to the king and his loyalty to his Stark family when a conflict arises between them. Maybe he discovers a dark secret within the Kingsguard that threatens the stability of the realm.
His main allies could include Davos Seaworth. Davos is a shrewd man and has a lot of experience in the political games of Westeros. He could offer Jon advice on how to navigate the court. Another possible ally could be Brienne of Tarth. She is a skilled warrior and shares Jon's sense of honor. They could fight side by side to protect the king. Also, Gendry could be an ally. He has his own strength and could be useful in battles to defend the king.