One great 'Robb Stark OC fanfiction' is 'The North's New Hope'. It has an original character who becomes an important ally to Robb. The story is full of political intrigue and battles, just like in the original 'Game of Thrones' setting.
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 great 'Game of Thrones Robb Stark OC fanfiction' is 'The Stark Heir'. It follows an original character who becomes an important ally to Robb Stark. The story has great character development and exciting battle scenes.
When writing, use vivid descriptions. Describe the cold of the North, the armor of Robb's soldiers, and the unique features of your OC. Also, create conflicts and resolutions. It could be that the OC's presence causes jealousy among Robb's men at first, but later they come to respect the OC's abilities. And don't forget to proofread for grammar and spelling errors.
One plot idea could be that the Baratheon OC is a long - lost relative who comes to claim a part of the Baratheon inheritance. Robb Stark gets involved as he has political interests in the Baratheon realm. They start off as rivals but gradually develop a mutual respect as they face common enemies.
First, do your research on Robb Stark's character. Know his traits, his goals, and his relationships in the original series. Then, create your original character with a clear backstory, personality, and motivation. Make sure the OC's story can fit into the world of Game of Thrones. For example, if your OC is a warrior, think about where they learned their skills and why they would side with Robb Stark.
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.