Well, an OC (Original Character) son of Bolton could be a really interesting addition to a fanfiction. He might be a cruel and calculating character, given the Bolton family's reputation in 'Game of Thrones'. He could be vying for power in the North, perhaps trying to outwit the Starks or other northern houses. Maybe he has some unique skills, like being an expert tracker or a master of psychological warfare.
He could be a character who is conflicted. On one hand, he has the Bolton blood which is known for its ruthlessness. But on the other hand, he might have some redeeming qualities. For example, he could fall in love with someone from a rival house and that could change his whole perspective. In the fanfiction world, he could be used to explore different storylines that weren't fully explored in the original series, like the inner workings of the Bolton family during times of peace in the North.
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 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.
Ramsey Bolton is often a central character in many fanfictions. His cruel and unpredictable nature makes for interesting plot twists. Writers like to explore his backstory or create scenarios where he meets his end in different ways.
One possibility could be that the oc, being the son of Drogo, has a claim to the Dothraki leadership. He could strive to unite the Dothraki clans in a different way than Daenerys did. Maybe he focuses on forging alliances with the other kingdoms based on mutual trade and protection rather than conquest.
I think it could be the blacksmith OC who has a strong moral code. For instance, an OC who refuses to make weapons for evil lords or sells his work only to those who he believes will use it for good. This type of character adds depth to the fanfiction and can be very appealing to readers who like complex and morally upright characters.
Perhaps the OC son has a special ability or gift that Jon Arryn was trying to protect him from being exploited for. After Jon Arryn's death, the son has to learn to control this ability while also dealing with the power struggles in King's Landing. He could be a key figure in the fight against the White Walkers, using his unique skills to help the living, all while trying to uphold his father's legacy.
In some House Bolton fanfiction, there could be a story about a Bolton heir who is secretly in love with a Stark. This would create a great deal of internal conflict, as the two houses are traditional enemies. The Bolton heir would have to decide between loyalty to family and love.
He could influence events by being a voice of reason in Cersei's court. If he's intelligent and perceptive, he might be able to persuade his mother to make more diplomatic decisions rather than always resorting to force.
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.
One possible plot could be the son growing up in the shadow of his mother's power-hungry ways. He might struggle with the expectations placed on him due to his family name. For example, he could be groomed to be a political pawn but secretly desires a more peaceful life away from the power struggles of King's Landing.