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.
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.
Jon Snow could potentially have allies like the Mormonts. They have always been loyal to the Starks. Maybe even some of the Dornish houses might side with him. They have their own reasons to oppose the current power structure in Westeros. And of course, his friends from the Night's Watch who left with him or believe in his cause could be his allies in this conquest. He might also gain the support of some of the maesters who see the value in his leadership for the future of Westeros.
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 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.
One aspect that fanfiction might focus on is how Val's wildling background interacts with Jon Snow's upbringing in the Night's Watch. Their cultural differences could lead to both conflict and a deeper understanding between them. Maybe in some fanfics, Val helps Jon Snow better understand the ways of the wildlings, which in turn gives him an edge in dealing with the threats beyond the Wall.
It could be that in this particular fanfiction, the writer has a different vision for Lucy's character development. Perhaps they think she can grow more independently outside of Fairy Tail. There might also be some plot twists that prevent her from joining, like she has a prior obligation to another group or person that conflicts with joining Fairy Tail.
Sansa Stark might be involved as well. Given her relationship with Jon Snow as his sister (though later revealed otherwise), she could play a role in protecting or being involved with the baby. Maybe she helps raise the baby in a world where Jon Snow is constantly facing threats.
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.
In this fanfiction, one of the key battles Jon Snow might participate in could be the Battle of the Whispering Wood. His skills as a fighter would be put to good use there, sneaking around and taking out enemies. Another could be a fictional battle near the Wall, where he defends against some new threat that has emerged during Robb's war.