One possible plot could be that after the marriage, Robb Stark has to deal with the complex Frey politics. He might try to use the union to strengthen his position in the North. Maybe he and his Frey bride gradually fall in love despite the initial political motives of the marriage.
It could lead to a major shift in the power dynamics of the story. Maybe it would prevent the Red Wedding or cause other unforeseen conflicts.
If Robb Stark married a Frey girl, it might have led to a more stable alliance between the Starks and the Freys. This could have affected the power dynamics in the story and altered the outcome of the war.
A possible plot could be that Rhaenys and Robb meet by chance, and despite the initial shock of their different backgrounds, they fall in love. Their marriage then becomes a symbol of hope for a more united Westeros. They might have to overcome the objections of their families and the political unrest that such a union could cause. It could also involve them learning about each other's traditions and forging a new path for their combined houses.
One consequence could be a shift in the power balance. The Mountain was a powerful fighter in service of certain lords. His death would leave a power vacuum among his allies. Robb Stark, on the other hand, would gain more notoriety. His reputation as a fearsome and capable leader would spread, making his enemies more cautious of him.
It could change it a great deal. In the original story, Robb's actions regarding the Freys led to the Red Wedding. But if he married a Frey, it would mean no Red Wedding. This would keep Robb alive and his army intact. It could also change the power dynamics in Westeros as the Stark - Frey alliance would be very strong.
The plot could center around a love story that overcomes many obstacles. They come from different regions with different traditions. Rhaenys might face challenges in adapting to the cold and harsh North, while Robb might have to deal with the expectations of his bannermen regarding this foreign bride. However, their love could grow as they learn about each other's strengths and weaknesses. They could start to see the similarities between their two families in terms of loyalty to their own, and this mutual understanding could form the basis of a strong relationship. Their marriage might also be a symbol of a new era of unity between the South and the North, with their children potentially being heirs to a more united Westeros.
One possible plot could be that they face a common enemy that neither the North nor the Night's Watch can ignore alone. Maybe a new and powerful wildling leader has emerged, uniting the tribes in a way that threatens all of Westeros. Jon, with his knowledge of the wildlings, and Robb, with his military leadership skills, join forces to combat this threat.
Well, if Robb Stark wins the war in the fanfiction, he would likely have a major impact on the political landscape. He could reshape the power dynamics, perhaps even expanding his influence beyond the North. He might also be in a position to avenge his father's death more thoroughly and bring stability to the realm. Additionally, his victory could lead to different character developments for those around him, like his mother or his bannermen.
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.
Well, in this fanfiction, Robb Stark might experience a spiritual awakening. He could be guided by the Old Gods, perhaps receiving visions or prophecies that would change his course of action in the war. Maybe he'd find ancient wisdom that gives him an edge against his enemies.