One possible ending is that Robb Stark becomes a great and respected king, ruling over a prosperous North. Another could be that he steps down after achieving victory and hands over the power to someone he trusts, perhaps to live a more peaceful life.
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.
If Robb won the war in a fanfic, it could lead to a completely different Westeros. He might establish a more just rule in the North and perhaps even expand his influence over other kingdoms. Maybe he'd form new alliances and bring about an era of peace and prosperity, unlike the chaos that followed in the original series.
In a fanfic where Robb wins the war, the Stark family's reputation would be greatly enhanced. Robb would be seen as a great leader. He might also start to reform the feudal system in Westeros. For instance, he could give more power to the smallfolk, which would be a revolutionary change. This could lead to a more democratic - like system in the long run. He would also have to deal with the remaining enemies who might be plotting revenge, so he would need to keep a strong military presence.
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.
Well, in the fanfiction world, Robb Stark might also get some unexpected help. Maybe there are some magical elements involved. A wizard or a sorceress could weaken the Mountain's strength or defenses. This would give Robb an edge in the fight. Then, with his courage and fighting skills, he could step in and slay the Mountain.
The Old Gods themselves would be crucial. They could be represented in various forms like the weirwood trees. And there might be some sort of priest or seer who serves as an intermediary between Robb and the Old Gods, guiding him through the spiritual experiences.
The Lannisters could be the main antagonists. They have a history of being power - hungry and have caused a lot of trouble for the Starks. In this fanfiction, they might be trying to expand their territory further north, which would prompt Robb and Jon to go to war to protect their homeland.
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.