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.
He could meet some of the last remaining Valyrian descendants. They might be hiding in the ruins, still holding on to their ancient knowledge and traditions.
He might meet descendants of the Valyrian people. These could be people who have managed to survive the cataclysm that befell Valyria and have unique knowledge about the place and its history.
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.
Jon Snow could discover ancient Valyrian magic. Since Valyria was a center of magic before its fall, he may stumble upon spells or artifacts that could change his fate or give him new powers. He might also meet other characters who have a connection to Valyria's past, and together they explore the ruins and uncover long - forgotten secrets about the once - great civilization.
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 possibility for 'jon goes to valyria fanfiction' is that Jon is sent on a quest to Valyria by Daenerys. She believes that there are resources or allies in Valyria that could strengthen their cause. Once in Valyria, Jon has to deal with the harsh and dangerous environment. There could be volcanic eruptions, poisoned air, and dangerous beasts. He also starts to have visions of the past, seeing how the Valyrians rose and fell, and he uses these insights to try and prevent similar disasters from happening in Westeros.
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.