There could be a fearless space captain as one of the main characters. He or she might be leading a group of rebels against a tyrannical interstellar regime, much like how some characters in 'Game of Thrones' fought against unjust rulers.
Maybe an alien princess who is next in line to rule a powerful planet. But there are others who want to usurp her position, so she has to navigate through political plots and battles, similar to the female characters in 'Game of Thrones' who had to fight for their claim to power.
I think Naruto would be like the central hero, similar to how Daenerys was in Game of Thrones. He wants to bring change and justice. Then there's Kakashi, who could be like a wise advisor, much like Tyrion Lannister in some ways. And Gaara, with his past of being an outcast, could be a character who rises above his circumstances, just like Theon Greyjoy did at some points.
One main character could be a genetically - enhanced human who has special abilities, like Arya Stark's skills in the original. She could be on a mission to avenge her family who were killed by a powerful space - faring clan. There could also be a powerful warlord from a distant star system, a bit like Ramsay Bolton, feared for his cruelty and thirst for power. The interactions between these diverse characters would drive the story forward in this space - based Game of Thrones fanfiction.
There could be a young, ambitious knight who has seen the injustices in the Seven Kingdoms and becomes a key figure in the uprising. Another might be an old and wise maester who secretly supports the uprising and uses his knowledge to help the rebels. Also, a strong - willed noblewoman who has her own reasons for wanting change could be one of the main characters.
There's no way to know exactly without reading it. However, in a 'Game of Thrones' - themed fanfic with 'no mercy' in the title, it's likely to have some of the fan - favorite characters in key roles. Maybe Arya Stark, with her assassin skills, could be a main character, showing no mercy to those on her kill list. Or it could be Cersei Lannister, being even more merciless in her quest for power and revenge than she was in the original series.
Cinderella would be a central figure. For the male lead, it could be a young and noble - hearted character like Robb Stark. He has the charm and leadership qualities similar to a prince. The stepmother could be a power - hungry noblewoman, like Cersei Lannister in some ways. The stepsisters could be her daughters, who are jealous of Cinderella's beauty and the attention she gets from the young lord. And then there could be other 'Game of Thrones' characters like Tyrion Lannister as a side character providing some wisdom and humor.
Daenerys could be a main character. She has leadership skills and her dragons could be a huge asset in a war - like scenario inspired by WWI. Another could be Jon Snow. His experience in leading the Night's Watch and fighting the White Walkers could translate into him being a key figure in this fanfiction's battles. Tyrion Lannister would also be a likely main character. His intelligence and ability to strategize would be very useful in a complex situation like a WWI - style conflict in Westeros.
Definitely the major world power countryballs like the United States, Russia, and China. They would likely play important roles due to their real - world influence in space exploration.
There might be a Stark character as well, since in 'Game of Thrones' the Starks are often associated with wolves. They could be intertwined with the Black Wolf's story, perhaps as its protector or someone it protects.
The main characters could also be entirely new characters introduced in the fanfic. The author might create someone who has a unique connection to the events of the original series and is given the task of changing things. However, it's also possible that classic villains like Cersei could be main characters if the fanfic is about giving her a second chance at ruling, perhaps with a different approach this time.
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.