Perhaps a young Roman centurion who gets lost and ends up in Westeros. He would bring his military training and Roman values with him. He could form alliances with some of the more honorable characters in 'Game of Thrones', like Ned Stark. Or it could be a female character from Westeros, a strong - willed lady, who is kidnapped and taken to the Roman Empire and has to fight for her freedom and find her way back home, interacting with Roman emperors and senators along the way.
One main character could be a maester from Westeros who is sent on a diplomatic mission to the Roman Empire equivalent in this fanfic world. He is intelligent and well - read, and his knowledge of different cultures and sciences could help him understand the Romans. Another could be a Roman gladiator who escapes and flees to Westeros, where he has to use his combat skills to survive in a land full of political turmoil and danger.
A possible main character could be a Westerosi noble who is intrigued by the Roman Empire and decides to journey there. He could be like Tyrion Lannister, smart and cunning, trying to navigate the new and strange world of the Romans. Another could be a Roman general who finds himself in Westeros and has to adapt to its different rules of war and power.
Well, it could be a story where the values of the Roman Empire, such as honor and loyalty in the military, are introduced to the characters in 'Game of Thrones'. Maybe a character from the Seven Kingdoms travels to a land inspired by the Roman Empire and brings back new ideas. Or perhaps there are gladiator - like battles in the arenas of Westeros, with characters from both worlds competing for glory. It would be a wild mix of the two universes.
The common people from the Roman Empire would face a lot of difficulties. The harsh winters, the constant threat of war, and the feudal system in Westeros are very different from what they are used to. But some might be able to use their skills in farming, building, or trading to carve out a living in this new and dangerous land.
A samurai - like character from Zipang could be a main character. They would be skilled in combat and bound by a strict code of honor. They could enter the fray in Westeros and become involved in the power struggles, perhaps even challenging the likes of Jaime Lannister in a duel of honor.
You could combine characters by having a Roman character end up in Westeros. For example, a Roman centurion could be transported to Westeros and find himself in the middle of the War of the Five Kings. He could use his military training to fight for one of the sides or try to find a way back home.
The Night King could be a central character in a sense. His actions and motives would drive much of the conflict. Bran Stark might also be a main character. His powers as the Three - Eyed Raven could be important in the battle. And don't forget Tyrion Lannister. His wits and strategic thinking could be vital in leading the defense against the enemy.
The owner of the company could be a main character. Maybe he's a shrewd merchant who has built the company from the ground up. Another could be the company's best fighter, a tough and battle - hardened warrior who protects the company's interests. And there could also be a young apprentice in the company, who is just learning the ropes but has a lot of potential and a different perspective on things.
There could be a group of main characters. One could be an old maester who has seen many things during the previous era and now serves as a mentor to the younger generation. A second could be a wildling who has decided to integrate into Westeros society but faces many challenges. And a third could be a young noblewoman who is disillusioned with the traditional role of women in Westeros and is fighting for more equality. Together, they could be involved in all sorts of adventures and political intrigue in the new era of 'Game of Thrones'.
One interesting crossover could be the political intrigue. In 'Game of Thrones', the power struggles between different houses are intense. In the Roman Empire, there were also power struggles among senators and the imperial family. So, a fanfiction could explore how the political tactics from one world would play out in the other. For example, the Roman concept of patron - client relationships might be applied to the relationships between lords and vassals in Westeros.
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.
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.