Well, the dragon would obviously be a main character in its own right. It could have its own personality and motives.
There might be other characters from the original series who play important roles. For example, Daenerys might be in the fanfic. Since she was associated with dragons in the original story, she could have an interesting dynamic with Jon Snow now that he's also linked to a dragon. Maybe they compete for the dragon's loyalty or work together to use the dragon's power.
Then there could be some new characters created specifically for the fanfic. Maybe a dragon trainer who helps Jon Snow understand his new connection with the dragon. This character could introduce new plot twists and conflicts, like having different ideas about how to handle the dragon compared to Jon Snow.
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.
Obviously, Jon Snow as a reborn dragon is the main character. But there could also be characters like Daenerys who might play an important role. She has her own connection with dragons, so her reaction to Jon's new form could be a significant part of the story.
Obviously, Jon Snow is a main character. He's usually depicted as having a special bond with the ice dragon. His character traits like his honor and leadership often play a big role in the story.
Tyrion Lannister could be involved. He was close to Daenerys and had a relationship with Jon as well. He might be the one to help protect the baby or offer his wisdom in raising the child. Another character could be Ser Davos. He was loyal to Jon and would surely be interested in safeguarding the future of Jon's heir.
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.
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.
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 popular 'Jon Snow Black Dragon' fanfic is 'The Bond of Ice and Dragon'. It tells a story where Jon Snow forms an unexpected connection with a black dragon. The story is filled with adventure and a touch of magic.
One common theme is power. The dragon gives Jon Snow a great deal of power, and these fics often explore how he wields it. Another theme is identity. Since Jon Snow has a complex identity, and the addition of a dragon connection further complicates it. There's also the theme of destiny. Fics may show that Jon Snow was always meant to have a connection with a dragon as part of his greater destiny in Westeros.