She could use her Faceless Men skills. Maybe she disguises herself as one of the serving staff. Since she has been trained to blend in and move unnoticed. She could then poison the wine or food that was meant for the main players in the Red Wedding. This way, she can stop the massacre before it even starts.
Yes, the Red Wedding in 'Game of Thrones' is said to be based on some real - life historical events. In history, there have been instances of so - called 'treacherous feasts' where supposed allies turned on each other during a time of supposed peace and celebration. These events often involved political intrigue and power struggles, much like in the fictional Red Wedding.
The Stark family would be in a much stronger position. Robb Stark would not be killed, which means the North would remain united under his leadership. This could potentially change the entire power dynamic in Westeros. The Boltons and Freys would be seen as traitors and might face severe consequences from the other houses.
The Red Wedding may be inspired by various historical events of treachery. For example, in some medieval European kingdoms, lords would invite their rivals to feasts and then turn on them. These events were often part of the power struggles within the nobility. The Red Wedding has a similar feel of a sudden and brutal shift from celebration to massacre for political motives.
The story would change in many ways. Robb's mother, Catelyn, might also have survived if he lived. Their combined presence would have a huge impact on the Stark family's standing. Robb could also have a different approach to dealing with the Lannisters. Instead of being on the defensive all the time, he could be more aggressive with his military campaigns, knowing that he has a second chance at life. Also, his relationship with his siblings, especially Sansa and Arya, who are in difficult situations in King's Landing, could be developed in new and interesting ways.
In the fanfic, it's possible that Jon Snow uses his warging ability. He might warg into a large and fearsome animal, like a direwolf or even a wild boar. He could then send this animal into the middle of the Red Wedding chaos, causing enough distraction for him to step in and stop the massacre. He could also use his connection with the Northmen present, appealing to their loyalty to House Stark and the North, convincing them to turn against the Freys and Boltons.
One possible plotline could be that Robb manages to escape with a small group of his most loyal men. They then go into hiding, perhaps in the North, and start planning their revenge against the Freys and Boltons. Robb might also try to gather more allies, like the remaining Stark bannermen who are still loyal to him.
In this fanfic, House Bolton would also suffer. Their alliance with House Frey was a key part of their plan to gain more power in the North. If Jon Snow stops the Red Wedding, their scheme fails. They would lose the support of many of their allies who were only in it for the chance to get on the winning side. For House Tully, they would be grateful to Jon Snow. Their family members were also at risk at the Red Wedding, and if Jon Snow saves them, it would strengthen the bond between House Tully and House Stark, potentially leading to future alliances and mutual support in the power struggles of Westeros.
Maybe the plotline focuses on the cultural differences that the OC brings to Sansa's wedding. For example, if the OC is from Dorne, they might introduce new traditions such as a different style of wedding ceremony or unique wedding gifts. This could cause some intrigue and also some conflict as the Northerners are not used to such things. Also, the OC could be a healer who discovers a plot to poison Sansa during the wedding and has to race against time to save her, all while falling in love with her charm and strength.
Maybe Castle realizes he can't live without Beckett and confesses his love at the last minute, which stops the wedding. For example, he could show up with some big revelation about their relationship that makes Beckett question going through with the wedding.