Yes, there might be. There are numerous fanfiction platforms out there like Archive of Our Own (AO3) where such stories could potentially exist. You can try searching on these platforms using relevant keywords to find it.
You could try Fanfiction.net as well. It's one of the oldest and most popular fanfiction platforms. There are a lot of works on there, so it may take some time to sift through, but it's a good option. Also, check out some niche fanfiction communities related to the specific fandom that these characters belong to. They might have their own repositories or links to relevant fanfictions.
One possible plotline could be that they meet in a secret location. Maybe Rhaenys is on a mission and Jon stumbles upon her. They initially distrust each other due to the complex political situation in Westeros. But as they start to talk, they realize they have more in common than they thought. They both have a sense of justice and a desire to see the realm in a better state. This could lead to them forming an alliance and working together to face the various threats in Westeros.
Well, clearly Jon Snow and Arianne Martell are the main characters. Jon Snow, who was thought to be a bastard son of Ned Stark but has a much more complex and mysterious heritage. He has proven himself to be a capable leader and warrior. Arianne Martell, on the other hand, is a strong - willed and politically astute woman from Dorne. She has her own ambitions and desires within the political landscape of Westeros.
A common plotline could revolve around the challenges they face from other houses. Since Jon Snow has his own supporters and enemies in the North, and Arianne has her situation in Dorne, there would be many who would try to undermine their union. This could lead to a series of adventures where they have to prove their love and the strength of their alliance. For example, they might have to deal with false rumors spread by rival houses to turn the public against them. They would then need to work together to quash these rumors and show that their marriage is beneficial for all of Westeros.
Well, given their different backgrounds, their relationship development could be a bit rocky at first. Jon's upbringing in the North makes him more down - to - earth and practical. Rhaenys, as a Targaryen, may have grander and more ambitious ideas. However, they could find common ground in their desire for a better Westeros. For example, Jon wants to protect the North and all of Westeros from the threats beyond the Wall, while Rhaenys may want to reclaim the Iron Throne for the Targaryens. As they work towards these goals together, they could start to respect and love each other.
A possible plot could be that Rhaenys and Robb meet by chance, and despite the initial shock of their different backgrounds, they fall in love. Their marriage then becomes a symbol of hope for a more united Westeros. They might have to overcome the objections of their families and the political unrest that such a union could cause. It could also involve them learning about each other's traditions and forging a new path for their combined houses.
One way to find fanfiction about Rhaenys Targaryen and Robb Stark getting married is to join relevant fan communities. For example, on Reddit, there are sub -reddits dedicated to different fandoms. You can post a request there asking if anyone has read or written such a fanfiction. People are usually quite helpful in these communities and might be able to point you in the right direction.
There could be a wise old wizard who gives Jon advice on how to hatch and take care of the dragon. Maybe there's also a rival of Jon's who is jealous of his discovery and tries to steal the dragon egg.
It's interesting because it can explore different aspects of their characters. For example, how their relationship might change if they were in different situations, like facing a common enemy or having to make difficult decisions together. It gives a new perspective on their individual personalities as well.