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 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.
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.
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.
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.
One of the main themes is family dynamics. The Lannisters have a complex family structure, and the OC has to find his place within it. Also, the theme of survival is prevalent. In the cut - throat world of Game of Thrones, the OC has to fight to stay alive and relevant. And then there's the theme of reputation, as the Lannister name comes with both advantages and burdens.
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.
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.
The OC son could start off as a naive and sheltered young man, but after Jon Arryn's death, he is forced to grow up quickly. He becomes more cynical and distrustful of those around him, especially the nobles in King's Landing.
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.