He could play the role of a hero. In a fanfiction, he may be the one to save the Iron Islands from a great threat, like an impending invasion or a natural disaster. He uses his wits and courage, which might be different from the typical brute force associated with the Greyjoys. He could also be a diplomat, forging new alliances for the Iron Islands, like making a pact with the Lannisters to gain wealth and protection in exchange for naval support.
He could be a new character created by the fanfiction writer. Maybe he's a strong and ambitious son of Greyjoy, with a desire to prove himself in the cut - throat world of Westeros, much like the other Greyjoy characters we know from the original series.
He could be a rebel. Tired of the Bolton family's traditional ways, he might try to change the family's reputation. He could be a force for good in the North, protecting the smallfolk from the usual Bolton brutality.
He could also play the role of a healer. In a world full of wars and diseases, a male OC with exceptional healing skills would be very valuable. He could be in a small town when a plague breaks out. He uses his knowledge of herbs and ancient healing techniques to save as many people as he can. This would make him a hero in the eyes of the common people, and perhaps catch the attention of a powerful noble who wants to use his skills for their own purposes.
The OC Greenseer could act as a guide. They might use their visions to lead characters to hidden treasures or safe havens. For instance, they could show a group of refugees the way to a secret valley where they can survive the winter and the war.
One common theme is loyalty. Since the Greyjoys are known for their strong sense of loyalty to their family and the Iron Islands. Another theme could be the struggle for power. The son of Greyjoy might be vying for a position of power within the islands or against other houses in the 'Game of Thrones' world.
They could be sisters in the fanfic. Ros, the older and more practical one, and Daisy, the younger and more idealistic OC. They could be from a noble but fallen house. Their story could revolve around trying to regain their family's honor. They might have to make difficult choices, like whether to side with Daenerys or Jon Snow in the fight for the Iron Throne.
Petyr Baelish often plays a devious role in OC fanfiction. He might be the mastermind behind many plots. For example, he could be using an OC to gain more power in King's Landing.
Margaery can also be a symbol of hope or change in fanfictions with a male OC. If the male OC is disillusioned with the current state of Westeros, Margaery could represent a new way forward. Her charm and intelligence can inspire the male OC to support her in her plans to reform the kingdom. Their relationship then becomes a driving force for the plot, as they strive to make Westeros a better place while also dealing with their own personal feelings for each other.
Yes, there are quite a few. You can start by checking out some popular fanfiction websites like Archive of Our Own. Just search for 'oc son of greyjoy' and you'll likely find a variety of stories with different themes and plotlines.
Rome could play the role of a mysterious foreign power. It might send envoys to the Seven Kingdoms to form alliances or gather information. This would add an extra layer of diplomatic intrigue to the story.