If we consider the Hulk's origin story in relation to the magic and mystery in Game of Thrones, there could be an interesting take. Maybe there's some sort of wild magic in Westeros that causes Bruce Banner - if he exists in this fanfiction world - to transform into the Hulk. And his transformation could be linked to the power struggles in the Seven Kingdoms. For example, if certain powerful mages or sorcerers wanted to use his strength for their own gain, or if he became an obstacle to their plans.
The Hulk might be seen as a sort of monster in the context of Game of Thrones. Just like how they have direwolves and dragons, the Hulk could be another mythical - but in this case, suddenly very real - creature. He could be shunned by some and perhaps worshipped as a god by others, depending on how different regions of Westeros react to his existence.
In a fanfiction scenario, Hulk could be a creation of the Children of the Forest. Long ago, they used their magic to create a being of great strength to protect them from the First Men. But as time passed, he was forgotten and buried. Then, with all the magic resurfacing in Westeros during the events of the series, Hulk is awakened. His presence would completely disrupt the power balance, as he doesn't care about the Iron Throne or the politics of the land, but just reacts to threats and challenges in his own brute - force way.
Hulk could be a protector of the Wall. Instead of the Night's Watch, he stands guard against the White Walkers. His strength would be a great asset against the undead hordes. He might not understand the politics of the Seven Kingdoms, but he knows when something is a threat. And the sight of a giant green monster fighting off the White Walkers would be an amazing addition to the Game of Thrones lore. He could also form an unlikely friendship with some of the members of the Night's Watch, like Jon Snow, who would respect his power and his simple sense of right and wrong.
The Hulk would probably have a very different dynamic with the various characters. With the more honorable characters like Ned Stark, they might initially be cautious but then try to reason with the Hulk or at least figure out how to co - exist with him. However, with more power - hungry characters like Cersei, she might see the Hulk as a threat to her rule and try to eliminate him.
In fanfiction, Aragorn could potentially be a king from a far - off land who arrives in Westeros. He might bring with him a different style of leadership and a sense of honor that is somewhat different from the lords and ladies of Westeros. He could get caught up in the power struggles, especially if he realizes the threat of the Iron Throne being misused. Maybe he would try to form alliances with the more noble houses to bring stability to the land.
She could fit in as a spy. Given her ability to deceive and her immortality, she could gather information for different factions. For instance, she could work for the Starks initially, but then switch sides if she saw more benefit elsewhere.
They could be a new faction. Samurai have a strict code of honor, much like the knights in Westeros. They might be seen as a threat initially, but could also form alliances. For example, they could side with the Starks who also value honor. Their unique fighting styles would be a novelty in the battles of Westeros.
A house gardener could fit in as a keeper of secrets. The gardens often have hidden corners and old trees that could hide things. He might stumble upon a hidden cache of weapons or a secret meeting place. His presence is often overlooked as he is just a gardener, so he can observe a lot without being suspected.
Kakashi could fit in as a foreign warrior. He could bring his ninja techniques which would seem like magic to the people of Westeros. His stealth and combat skills would make him a valuable asset or a feared opponent in the battles and power plays of the Seven Kingdoms.
Smaug could potentially be a part of the magical elements in the 'Game of Thrones' fanfiction. He could be connected to the old magic that once existed in Westeros. Maybe his presence awakens other dormant magical forces. His actions could also influence the political landscape. For example, if he attacks a certain region, the lords there would have to divert their resources from the power struggles to deal with the dragon threat, which in turn could give an advantage to other houses.
Legolas could be an outcast in the Game of Thrones world at first. But as he gets to know the people and their struggles, he decides to get involved. He could start a new movement, based on elven values like harmony with nature and loyalty. His actions could inspire characters like Jon Snow to think differently about leadership and ruling, and together they could work towards a more just and sustainable Westeros.