One common theme is redemption. Jon often seeks revenge not just for the sake of it, but also to redeem himself or the honor of those he cared about. Another theme is power. He usually gains some form of power, whether it's political, magical or in terms of influence, which he uses to exact his revenge.
Well, there's this fanfiction titled 'Revenge of the Bastard'. In it, Jon Snow uses his skills and knowledge gained from his time at the Wall and beyond to take revenge on his enemies. He forms unexpected alliances and outsmarts his foes. It's really exciting to read as it combines elements of mystery, action, and of course, revenge. It gives a new perspective on Jon's character and his capabilities.
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 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.
Daenerys Targaryen is often paired with Jon Snow in these fanfictions. Their connection in the original series, along with their both being strong leaders, makes for an interesting dynamic. They could rule Westeros together in a new era of peace and prosperity after Jon's rebirth.
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.
He could meet ancient Valyrian dragonlords. These dragonlords might still have some connection to their dragons, even if the dragons are long gone or in hiding. Another possibility is meeting the descendants of the original Valyrian slaves who have their own unique stories and skills.
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.