Another possible alliance for the Spartan in Westeros could be with the Night's Watch. The Night's Watch is all about defending the realm from threats, and Spartans are all about defending their homeland. The Spartan's discipline and combat skills would be a great asset to the Night's Watch in their fight against the White Walkers. Plus, the Spartan might be drawn to the noble cause of the Night's Watch, which is to protect all of Westeros.
Percy might form an alliance with the Greyjoys. Given his connection to the sea, they would see the value in having him on their side. They could use his powers to gain an advantage in their naval campaigns. Another possibility is an alliance with the Starks. The Starks are known for their honor, and Percy, being a hero in his own right, would likely be drawn to them. Plus, they could use his powers to protect their land from floods or droughts.
You might expect exciting battles and power struggles. Spartans are known for their warrior skills, and Westeros is full of political intrigue and warfare.
A Spartan would likely find it tough to fit into Westeros' social hierarchy at first. They come from a society where merit and combat ability are highly valued. In Westeros, birthright often determines one's position. However, their combat skills might earn them respect among the military. For example, they could quickly rise through the ranks in a lord's army.
Another possible alliance for Naruto in Marvel fanfiction could be with the Guardians of the Galaxy. His immortality would give him the confidence to explore the cosmos with them. He could use his chakra to help power up their ship or provide combat support during space battles.
He could potentially form an alliance with the Starks. The Starks value honor and loyalty, and Madara, with his own sense of loyalty to his own ideals (albeit in a rather extreme way in his original story), might find common ground. He could offer his combat skills to help them in their battles against their enemies. Another possibility is an alliance with the Targaryens. The Targaryens have a connection to magic and dragons, and Madara's own powerful and somewhat magical abilities could make for an interesting partnership. He might see value in their claim to the Iron Throne and support them in exchange for certain power and privileges.
Klaus Mikaelson could form an alliance with Daenerys Targaryen. Daenerys is known for being open to different beings and powers, and Klaus' strength could be of great use to her in her quest for the Iron Throne. Also, they both have a certain air of royalty about them.
Some of the Bleach captains could potentially ally with the angels. Considering their own sense of justice and the fact that they fight against evil in their own world, they might find common ground with the angels in the Highschool DxD setting. This alliance could be based on mutual protection and the goal of maintaining peace in the supernatural realm.
Draco might form an alliance with the Lannisters. They are both power - hungry and cunning. The Lannisters' wealth and influence would attract Draco, and they might see his wizarding abilities as an asset. He could also form an alliance with the Tyrells. The Tyrells are known for their political savvy, and Draco could use their connections to further his own goals in Westeros.
Superman might also form an alliance with some of the more noble and freedom - seeking human kingdoms. These kingdoms might see him as a way to bring peace and stability to their lands. For example, if a kingdom is being attacked by bandits or darkspawn, Superman could quickly come to their rescue. In return, they could offer him a place to stay and resources, and together they could work towards a more just and prosperous Thedas.
Well, in a 'A Spartan in Westeros' fanfic, one key plot element could be the Spartan's adaptation to the unique political landscape of Westeros. They might bring their own sense of honor and combat skills which could disrupt the existing power dynamics. For example, if they side with a particular house, they could change the course of battles with their superior fighting techniques.