In a fanfiction, Robb Stark's twin brother might be a seer. He has visions of the future that are different from Bran's. While Robb is fighting his wars, his twin is trying to use his gift to prevent disasters. For example, he might see the Red Wedding in advance and try to warn Robb. However, due to the chaos of the war and disbelief from others, his warnings go unheeded. This could lead to a story full of regret and a new determination to avenge Robb's death and change the fate of the Stark family.
One possible plot could be that Robb Stark manages to avoid the Red Wedding trap. Maybe he receives a tip - off from an unexpected source, like a disgruntled Frey servant. With this knowledge, he could turn the tables on his enemies. He might use his military strategy skills to ambush the Freys instead, and this could lead to a major shift in the power dynamics in the North, with Robb emerging as an even stronger leader.
I recommend 'Robb Stark: The Lost Legacy'. This fanfiction focuses on an alternate timeline where Robb survives. It's interesting as it explores how he would have changed the course of the story. It has some great character development and interactions with other key characters from the series.
In a fanfiction, their relationship could be complex. Maybe there is some jealousy between them. For instance, Robb might be seen as the more natural leader by their father, which could make the twin brother feel left out. But at the same time, they love each other deeply. When Robb is faced with difficult decisions, his twin brother is the one he turns to for advice, even if they don't always agree.
Another reason is that Robb Stark's story ended tragically in the original series. Fans are drawn to fanfiction as a way to rewrite his story, give him a better ending, or just see different interpretations of his character. It allows for a lot of creativity and speculation among fans.
Well, the plot could be that Hinata's twin brother is sent on a secret mission. He discovers a conspiracy within the village that could threaten the Hyuga clan. He has to gather evidence and allies, and Naruto could be one of the key allies. They might have to fight against some rogue ninja who are part of the conspiracy.
One great 'Game of Thrones Robb Stark OC fanfiction' is 'The Stark Heir'. It follows an original character who becomes an important ally to Robb Stark. The story has great character development and exciting battle scenes.
Another fanfiction could be about Jon Snow (who is later revealed to be Ned Stark's son). In this story, he could fully embrace his Targaryen heritage much earlier. He might form an alliance with Daenerys not as an equal but as a leader, using his knowledge of the North and the loyalty of the Night's Watch to help her claim the Iron Throne in a more peaceful way.
The twin brother might befriend Tyrion. In the original story, Tyrion is constantly undermined by Joffrey. But the twin could see Tyrion's intelligence and value. Together, they could form a powerful alliance against Cersei and Joffrey. They could work towards a more just rule in King's Landing, reforming the laws and improving the lives of the common people. This would completely shift the power dynamics within the Lannister family and in King's Landing as a whole.
One possible plot could be that Joffrey's twin brother is the complete opposite in nature. While Joffrey is cruel and tyrannical, his twin is kind - hearted and wise. He could secretly try to undo the damage Joffrey does, like secretly helping the common people that Joffrey oppresses. He might also try to influence his mother, Cersei, to be more merciful, which would create a lot of internal family drama as Cersei is so protective of Joffrey.
Perhaps Harry is a Stark and the cave is a place of prophecy. He enters the cave and sees visions of the future of Westeros, including the downfall of his family if certain actions are not taken. He then uses his knowledge from his previous life (if we assume some sort of reincarnation or transfer from his Harry Potter world) and his Stark upbringing to change the course of events. He might form alliances with unlikely characters, train in new forms of combat or magic found in the cave, and ultimately strive to put a Stark on the Iron Throne, all while trying to keep his family alive and the realm in balance.