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.
Stark's fate varies across different comic arcs. Sometimes he succumbs to powerful enemies or circumstances, but there are also instances where he escapes death and remains a prominent figure.
Fanfiction delves into Robb Stark's smartness in multiple aspects. His intelligence can be portrayed in his ability to learn from his mistakes. In some stories, after a defeat or a setback, he analyzes what went wrong and comes up with a new and better plan. His smart side is also shown in his interactions with different characters. He might be able to sense the true intentions of those around him, be it friends or foes. For example, he could see through the false loyalty of a bannerman or recognize the value of an unexpected ally. This way, fanfiction expands on the character of Robb Stark as a smart and capable leader.
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.
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.
In certain comic series, Tony Stark's fate involves death. However, the comic world is complex, and his story can vary depending on the writer and the context of the story.