Merlin could side with Jon Snow. Jon is a man of honor, much like Merlin himself. They both have a sense of duty towards protecting the realm. Merlin could use his magic to help Jon in his battles against the White Walkers, for instance, by creating magical barriers or enhancing the strength of Jon's army.
There's also a possibility that Itachi would remain neutral for a long time. He would observe the different players in the Game of Thrones, much like he observed the events in the Naruto world. Only when he sees a real threat to the greater good would he choose a side.
Well, Harry Potter would be a very different character in the Game of Thrones world. He'd probably have to deal with the complex politics there. Maybe he'd use his magic to outwit the scheming lords and ladies. He might also form alliances with some of the more honorable characters like Jon Snow. Magic could be seen as a powerful but also very dangerous tool in Westeros, so he'd have to be careful not to draw too much attention to himself.
If Darth Vader were in the Game of Thrones universe in a fanfic, he'd likely be a major disruptor. He could easily overpower most of the characters physically. However, the political intrigue might initially baffle him. But his strategic mind from the Star Wars universe could soon adapt. For example, he could form alliances with certain houses, perhaps using his power to intimidate them into cooperation, all while trying to achieve his own goals, whatever they might be in this new and strange world.
The Dragonborn could potentially fit into the Game of Thrones universe in several ways. One way is through their dragon - related abilities. In Game of Thrones, dragons are a powerful and rare force. The Dragonborn's natural affinity for dragons could make them a valuable ally or a feared enemy. They could perhaps form alliances with the Targaryens, who also have a strong connection to dragons. Another aspect could be their combat skills. The Dragonborn is often a skilled warrior, and in the brutal world of Game of Thrones, such skills would be highly sought after.
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.
One idea could be Merlin being a hidden advisor to one of the major houses in Westeros. He uses his magic secretly to influence events and protect the realm, much like he did in the Merlin series but in the Game of Thrones setting.
They'd change the power dynamics. Their skills in combat and their strategic thinking would make them a force to be reckoned with. They could be hired by different houses as mercenaries.
The Stark family would be in a much stronger position. Robb Stark would not be killed, which means the North would remain united under his leadership. This could potentially change the entire power dynamic in Westeros. The Boltons and Freys would be seen as traitors and might face severe consequences from the other houses.
In a fanfic, the Assassin's Creed elements could create some really interesting storylines. The idea of a hidden brotherhood operating in the shadows of Westeros would add a new layer of mystery. They could have their own goals, such as protecting a certain bloodline or preventing the misuse of magic. And their technology, like the hidden blades and the use of smoke bombs, would be a novelty in the medieval - like setting of Game of Thrones. This could lead to some really cool scenes where they use their gadgets to outwit their enemies and make their mark in the complex world of Westeros.