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.
Among the common people, it could be a cause for celebration in some areas. If the fanfiction portrays their marriage as a union for peace and prosperity, the people might welcome it. They would hope that the combined strength of the Starks and the Targaryens would mean better protection from threats such as bandits and invaders. Among the nobles, there would be a lot of political maneuvering. Some might try to use their marriage to their own advantage, while others might fear that they would be left out of the new power structure that would surely form as a result of this union. For example, smaller houses might try to ally themselves quickly with the couple to gain favor.
The plot could center around a love story that overcomes many obstacles. They come from different regions with different traditions. Rhaenys might face challenges in adapting to the cold and harsh North, while Robb might have to deal with the expectations of his bannermen regarding this foreign bride. However, their love could grow as they learn about each other's strengths and weaknesses. They could start to see the similarities between their two families in terms of loyalty to their own, and this mutual understanding could form the basis of a strong relationship. Their marriage might also be a symbol of a new era of unity between the South and the North, with their children potentially being heirs to a more united Westeros.
One way their storylines could intersect is through a political alliance. Robb Stark, being a key figure in the North, could seek Daenerys' help with her dragons to fight against their common enemies, like the Lannisters. Maybe Robb hears of Daenerys' power and decides to send an envoy to her. This could lead to a meeting between them and a negotiation of terms for an alliance.
They could have a friendship based on mutual respect. Robb respects Daenerys for her strength and her fight for justice in Essos, while Daenerys respects Robb for his leadership in the North. They might exchange stories of their struggles and learn from each other.
Yes, there are likely fanfictions out there with this concept. Fanfiction writers often create crossovers between different fictional universes like Harry Potter and Game of Thrones (where the Targaryens are from). You can search on popular fanfiction websites such as Archive of Our Own or FanFiction.net to find such stories.
One possible plot could be that after the marriage, Robb Stark has to deal with the complex Frey politics. He might try to use the union to strengthen his position in the North. Maybe he and his Frey bride gradually fall in love despite the initial political motives of the marriage.
If Robb Stark married a Frey girl, it might have led to a more stable alliance between the Starks and the Freys. This could have affected the power dynamics in the story and altered the outcome of the war.