One possible fanfiction could be where Jon Snow stands up to the Night King directly. He might use his skills and knowledge of the enemy to taunt the Night King, saying that he will never succeed in his dark plans. This shows his courage and determination to protect the realm.
This theme can also explore the idea of self - discovery. As Jon stands up for himself, he might learn more about his own values and limits. In fanfiction, he could be in a situation where he has to choose between his loyalty to different groups. By standing up for himself, he figures out where his true loyalties lie. It's a journey of self - realization within the context of a fictional world.
I haven't read this story in detail, but generally, it might be about a winner in a certain context who stands alone. Maybe in a competitive situation like a business or a game, the person who wins has to bear the solitude that comes with victory, such as losing some friends or facing envy from others.
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.
Daenerys Targaryen could be one to help. Since she has experience with dragons, she could give Jon Snow advice on how to care for the egg and hatch it.
It could be a young and untainted Frey. Maybe a niece or a cousin of the more infamous Freys. One who has not been involved in the past atrocities committed by her family and has a more noble heart.
Daenerys Targaryen is often paired with Jon Snow in such fanfictions. Their combined power and the potential for a different kind of relationship makes it a popular choice among writers. Another character is Arya Stark. Their sibling bond can be explored in new ways when Jon is reincarnated, like Arya helping Jon with her assassin skills in a different plot line.
The North would surely support him. They have always seen him as one of their own and a great leader. Also, the wildlings might back him, considering he has a history of working with them and understanding their plight. Davos Seaworth would most likely be on his side as well, given his loyalty to Jon in the past.