One possibility is that Jon Snow uses his new Faceless Man skills to infiltrate the Boltons' camp undetected. He could change his face to look like one of their soldiers and gather crucial intelligence for the Starks.
One great fanfiction could be 'Jon Snow's New Path'. In this story, Jon Snow discovers his connection to the Faceless Men in a very unexpected way. It delves deep into his struggle with his new identity and the powers that come with it. The author does a wonderful job of blending the mystery of the Faceless Men with Jon Snow's own character arc from the original series.
The combination of Jon Snow's character and the mystery of the Faceless Men. Jon Snow is a complex and beloved character from the original series. When paired with the enigma of the Faceless Men, it creates a lot of potential for interesting stories. His moral code and their amoral nature can lead to great internal and external conflicts.
As a Faceless Man, Jon Snow could have a different role in the fight against the White Walkers. Instead of leading an army openly, he could use his shape - shifting abilities to spy on the enemy. He could blend in with the White Walkers' ranks and find out their weaknesses, which would be a huge advantage for the living.
He could be recruited by the Faceless Men. Maybe they saw potential in him, perhaps his skills or his thirst for a new identity led them to approach him and offer him the chance to become one of them.
He could be a wizard who made a deal with a powerful magical entity. In exchange for great power, he lost his face. Now, he appears in the story of Harry Potter, and his presence might be a sign of a greater, more complex plot involving power struggles within the wizarding world that Harry will eventually get caught up in.
One possible plotline could be Tjon Snow using his new Faceless Man skills to infiltrate enemy strongholds. He might change his appearance to gather intelligence and undermine his foes from within.
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.
One idea could be Arya using her Faceless Man skills to avenge the wrongs done to her family. She could infiltrate the enemies' strongholds unnoticed and take them out one by one.
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.