The main character could be a former servant of the Reynes who has been waiting for their return. This person has a deep loyalty to the family and has been gathering information and resources in their absence. There could also be a character from the Faith of the Seven who is either for or against the Reynes' return, depending on how he interprets the teachings in relation to the Reynes' past actions.
One possible plot could be that the Reynes return with a newfound power. Maybe they have allied with some mysterious group during their absence. They could come back to claim their old lands and seek revenge on those who wronged them in the past. Their return might also stir up the political situation in Westeros, causing new alliances and rivalries to form.
There might be a new character introduced in the fanfic. This character could be a commoner who has a special connection with the dragon. For example, a shepherd who once saved the dragon when it was a hatchling and now the dragon has returned to repay the kindness. This character then gets involved in the high - stakes politics of Westeros because of their relationship with the dragon. Another main character could be a powerful lord or lady who sees the dragon's return as a threat to their power and will do anything to stop it or control it for their own gain.
Arya Stark could be a main character. She could use her skills as a Faceless Man to infiltrate the enemy's camp before a big battle and gather intelligence. Her ability to change her appearance would give her an edge in spying and sabotage operations during the battle.
It could be Arya Stark as the bodyguard. Given her skills in combat and disguise, she would make a great protector. She could be protecting someone like Bran Stark as he continues to develop his powers and is a target because of his abilities.
The children of the original characters would be likely main characters. For example, Jon Snow's (if he has any) descendants. They could inherit his sense of honor and face new challenges in the post - war Westeros.
You could have a main character who is a commoner. Maybe a peasant girl who witnesses the fall of a great house and has to navigate the chaos that follows. She could get involved with the various factions vying for power, and through her eyes, we see the events of the prequel unfold. This would give a different perspective compared to the noble - centered stories we often see in the 'Game of Thrones' universe.
Gendry could also play a major role. As a skilled blacksmith and a bastard like Jon, he has the strength and the will to fight for change. He could use his skills to arm the revolutionaries and his knowledge of the common people's plight to inspire them to rise up against the unjust noble rule.
Jon Snow would also make a great main character. He could be on a mission to unite the different houses' spaceships against a common threat, like an alien race or a powerful space - faring pirate group. His leadership skills and sense of justice would be put to the test in this new setting.
Daenerys Targaryen would be an obvious choice. Since she was the 'Mother of Dragons' in the original series, her return or a continuation of her story in a fanfiction where the dragons come back would be very fitting. She could have new adventures and use the dragons to reclaim her rightful place on the Iron Throne or to bring justice to Westeros.
In a fanfiction about the return of dragons in 'Game of Thrones', one possibility could be that the dragons bring a new era of balance to Westeros. They could help in rebuilding the kingdoms that were ravaged by war. For example, Drogon could use his fire to clear the rubble in King's Landing and then help plant new crops with his heat, making the land fertile again. Their return might also force the remaining lords and ladies to put aside their petty squabbles and unite under a new banner, perhaps one that represents the power and majesty of the dragons.