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.
You can start by looking on Archive of Our Own (AO3). It has a vast collection of Game of Thrones fanfictions, and there are likely to be many that feature a female Robb Stark and Theon. Just use the search function and type in relevant keywords like 'female Robb Stark' and 'Theon'.
Another theme could be a romantic relationship. Some fanfictions might develop a love story between them, which often includes elements of forbidden love as their families and loyalties are complex in the original 'Game of Thrones' context. It can be interesting to see how the writers navigate this.
Perhaps a female Robb Stark and Theon are forced into an arranged marriage as part of a political alliance between the remaining Starks and some of Theon's supporters. At first, they resent each other due to their past differences. But as they start to understand each other's motives and the greater good they are working towards, they fall in love. They then have to deal with the internal politics of their newfound union and fend off enemies who want to break them apart.
Another place could be FanFiction.net. However, the quality can vary a great deal. You might need to sift through quite a few stories to find the really good ones about female Robb Stark and Theon. But it has been around for a long time and has a huge collection.
If we consider a female Robb Stark in fanfiction, their relationship could develop more romantically compared to the canon. Since a female Robb might bring different emotional and social dynamics. Theon, who is often in search of acceptance, could be drawn to her in a more intense way. Their relationship could be less about power struggles and more about mutual support and affection, which would be a big departure from how their relationship was portrayed in the original story.
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.