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.
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.
In fanfiction about House Reyne, we could see alternate storylines. For example, what if the Reyne's had managed to avoid their brutal end at the hands of the Lannisters? Maybe they could have formed an alliance with another powerful house against the Lannisters. Their sigil, the red lion, could be a symbol of their resurgence in these fictional tales.
I'm not sure which ones are the most popular specifically, but there are likely some out there. Fanfiction websites are full of stories, and Domeric Bolton is an interesting enough character that some writers would have been drawn to him.
One interesting fanfiction could be about Domeric Bolton's early life and his relationship with his father. It might explore how he was raised in the harsh Bolton environment and his dreams that were different from the family's brutal ways.
One great 'Game of Thrones House Arryn fanfiction' is 'The Arryn Ascent'. It focuses on the rebuilding of House Arryn's power. The author beautifully describes the Vale's landscapes and the political maneuverings within the house.
One idea for a new house in a 'Game of Thrones' fanfiction could be House Frostwood. They could be from a far northern region, beyond the Wall. Their sigil could be a white wolf on a blue field, symbolizing their connection to the harsh, cold environment and the wild wolves that roam there. Their motto might be 'In Ice, We Endure'.
Another plotline might involve a Reyne character getting involved in the political intrigue at King's Landing. They could use their knowledge of the Lannisters' past deeds against House Reyne to manipulate the situation and gain power for themselves and their house.