The death of Lord Ned Stark was significant because it showed that no one, not even an honorable man like him, was safe in the cut - throat world of Westeros politics. It made the readers realize that the story was not going to follow the typical hero - always - survives trope. His death also led to his daughters' individual growth and transformation in different and difficult ways.
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.
One common theme is leadership. Robb Stark is a leader, so many fanfics explore how he leads his men, makes strategies, and deals with political situations.
The most memorable part was probably their marriage. It was a bold move by Robb against the expectations of his lords and the political system. It showed his determination to follow his heart and be with the woman he loved, Talisa.
There's a fanfic titled 'Robb Stark's Sister: A Tale with Theon'. In this one, the female Robb Stark character is really strong - willed. She has her own ideas about the future of Winterfell and her relationship with Theon is full of twists and turns. The author does a great job of showing how their different backgrounds shape their interactions. It's an interesting read if you like character - driven stories.