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.
It's possible that he meets other lost Westerosi adventurers who, like him, were drawn to Valyria in search of something. They could form an alliance or be rivals. He might also run into some strange and magical creatures that are native to Valyria and have a form of intelligence. For example, some sort of sentient fire - based entities that guard the ancient Valyrian ruins.
In this fanfiction, perhaps Jon Snow goes to Valyria to discover ancient secrets about his heritage. Maybe he finds lost Valyrian steel - making techniques or uncovers hidden knowledge about the White Walkers' origin. It could also be that he meets some long - lost relatives or discovers a new power source there.
In some fanfictions, if Jon Snow goes to Old Valyria, he may meet characters from the past. He could interact with ancient Valyrian families or even those who were involved in the events that led to the Doom. This could give him insights into the history of the world and how it has shaped the present, and potentially give him ideas on how to deal with the threats in Westeros.
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.
Maybe to seek answers. The secrets of his parentage might be somehow linked to Old Valyria. He could believe that going there would give him a better understanding of who he really is and his place in the world.
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.
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.
Well, the dragon would obviously be a main character in its own right. It could have its own personality and motives.
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.
Well, it could be about Jon Snow leading his men into a great battle against the White Walkers. Maybe he discovers new strategies or forms unexpected alliances during the war.