Common themes in Jon Snow Night's Watch fanfiction include leadership. Jon Snow is often depicted as a leader within the Night's Watch, facing challenges and making difficult decisions. Friendship is also important, his relationships with Samwell Tarly and other members are explored. And the theme of identity, as he is a Stark but also a member of the Night's Watch, and how he reconciles these two parts of himself in different fanfictions.
The White Walkers can still be the main antagonist even if he leaves the Night's Watch. After all, they are a huge threat to all of Westeros. Jon might think he can fight them better outside the Night's Watch, but they are constantly advancing and could target him specifically for leaving his post. Maybe they see him as a key figure in the coming war and want to eliminate him before he can gather enough strength to oppose them.
In some fanfictions, he might leave due to a love interest. He could meet someone who makes him question his vows to the Night's Watch. This person could be from a different region or have a special connection to him. For example, she might be a noblewoman with a hidden agenda, but Jon is drawn to her and decides to leave the Night's Watch to be with her and get involved in the political intrigue of the Seven Kingdoms.
One great Jon Snow Night's Watch fanfiction is 'The Trials of Jon Snow at the Wall'. It delves deep into his struggles and relationships within the Night's Watch. Another is 'Jon Snow: Defender of the Wall' which focuses on his heroic deeds in protecting the Wall from various threats. And 'A Different Path for Jon Snow at the Night's Watch' offers an alternate take on his journey in the Night's Watch, showing how different decisions could have led to different outcomes.
One possible fanfiction could be that Jon Snow discovers a long - lost magic that can actually communicate with the Night King. Instead of fighting, they have a series of intense conversations that make Jon understand the Night King's motives, which are not as simple as just bringing eternal winter. It could be that the Night King was once a hero wronged by the old magic and is now on a path of vengeance in a very twisted way.
It's possible that he has a different vision for his life. He might have seen a different future for himself, one where he doesn't have to be bound by the rules and isolation of the Night's Watch. He may want to be more involved in the politics and power struggles of the Seven Kingdoms, rather than being on the Wall.
The wildlings might be his allies. After all, Jon Snow has had interactions with them before. They are fierce fighters and could fight alongside him against a common enemy.
He might go back to Winterfell and claim his rightful place as a Stark. This would change the whole power dynamic in the North and also his relationship with his siblings.
In this fanfiction, Jon Snow's character could develop in several ways. Firstly, his relationships with other characters might change. He may form stronger bonds with his fellow soldiers, learning to trust them more. Secondly, his understanding of politics and power could grow. As he is involved in Robb's war, he witnesses the political maneuverings that go on behind the scenes. This could make him more cynical or more determined to fight for justice, depending on how he interprets these events. Moreover, his combat skills are likely to improve, and this newfound strength could give him a different perspective on his own capabilities and his place in the world.
In this fanfiction scenario, Jon Snow hatching a dragon would completely transform his relationship with Daenerys. At first, she might be in awe of the fact that Jon, a man she is already attracted to, has this new and powerful connection with a dragon. But as time goes on, power dynamics could shift. The dragon could act as a sort of mediator between them, or it could drive a wedge. If the dragon has a will of its own and favors Jon over Daenerys, it could cause a rift. However, if they can learn to work together with their dragons, they could become an unstoppable force in the fight against their common enemies.