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.
Robb winning the war in the fanfic might be due to his ability to gain more support from the common people. If he promised fairer rule and actually delivered on it, more people would be willing to fight for him. He could also have some unexpected help, like a powerful but previously unknown group coming to his aid.
It would change the storyline drastically. For one, many characters who died in the original story might survive. Robb's victory could mean a different fate for Sansa and Arya, as he would have more power to protect his family. Also, the political landscape would be very different. The Lannisters would be in a much weaker position, and that would affect how they interact with other houses.
One great fanfic could be where Robb finds Arya in a small village. He's overjoyed to see his sister alive. The story might focus on their emotional reunion and how they plan to reunite with the rest of their family. It could also include some action scenes as they face threats on their journey back to safety.
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.
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.
It would be a symbol of hope. In the dark and brutal world of Game of Thrones, family reunions are rare and precious. For Robb to find Arya, it means that a part of their family can be whole again. It also gives both characters a new sense of purpose. Robb now has another reason to fight and survive, to protect his sister. And Arya, who has been alone for so long, has someone she can truly trust again.