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.
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.
In fanfiction, Jon Snow could avenge the Red Wedding by forging new alliances. He might seek out the support of powerful magic - users or forgotten kingdoms. With their help, he could bring a supernatural vengeance upon those who committed the atrocities at the Red Wedding. Maybe he uses magic to curse their lands, or summons some mythical creatures to attack their strongholds. This way, he not only gets revenge but also makes it a memorable and fitting punishment for the cruel act that was the Red Wedding.
One way is through strategic warfare. He plans meticulously and attacks the enemies' weakest points. Another could be by using his Stark identity to rally the North against the traitors. He might also use his newfound knowledge from his experiences, like his time with the Wildlings, to outwit his foes.
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.
In this fanfic, Jon Snow could be a special Pokemon ranger. He is sent on a mission to a region that has been affected by a strange phenomenon that is making Pokemon act aggressively. Jon, with his experience in handling difficult situations in Westeros, uses his wisdom and combat skills. He forms a bond with a Growlithe, which he names Ghost after his direwolf. Together, they work to solve the mystery and restore peace to the Pokemon region.
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.
The main characters are Castle and Beckett, obviously. Castle is likely to be the one who intervenes in Beckett's wedding. Beckett is the one whose wedding is being stopped.
It depends on the specific fanfic. There are countless fanfictions out there, and while some may choose to give Jon Snow dragons to enhance his character or create a more fantastical story, others may stick closer to the original lore and not include such a plot element. So, you really have to dig into different fanfics to find out if he has dragons in them.