Well, obviously the OC king is a main character. He's the one around whom the whole story revolves. There might also be his advisors, who could be a mix of old and wise men from established houses, as well as some new faces with fresh ideas. His enemies are also important characters. They could be rival claimants to the throne, or lords who oppose his new ideas for Westeros. And don't forget about the love interest, if there is one. She could be a noble lady from a powerful house, or perhaps a mysterious woman with her own secrets and powers.
Well, at the beginning of 'Game of Thrones', the King of Westeros is Robert Baratheon. He came to power after the Rebellion against the Targaryens. He was a bit of a hedonist, liked his wine and women a great deal. His claim to the throne was through his distant Targaryen bloodline on his mother's side. He was a big, strong man in his youth but had let himself go a bit by the start of the story, which was part of his downfall in a way.
At the start of 'Game of Thrones', the King was Robert Baratheon. He had won the throne through war, defeating the Targaryens. However, his court was full of political intrigue. His relationship with his queen, Cersei, was strained as she had her own Lannister family interests. Also, he was in debt to the Lannisters which gave them a lot of influence over him. His excessive drinking and womanizing were starting to take a toll on his health and his ability to rule effectively.
Well, one possibility is that Naruto uses his ninja skills in the world of Westeros. He might use his shadow clone jutsu to confuse his enemies in battles against the knights or rival houses. Maybe he forms an alliance with a particular house using his charisma, like he did with his friends in the Naruto world.
Another plot could involve the OC king's relationships. He might have an unexpected alliance with a powerful but previously hostile house through a marriage of convenience. This could lead to a whole new power dynamic in Westeros as they combine their forces. Or perhaps he has a secret friendship with a commoner who becomes his most trusted advisor, which goes against the traditional norms of the land.
One possible plot could be that Harry Potter, through some magical accident or powerful spell, is transported to Westeros. He might use his magical abilities to quickly rise in power, perhaps by impressing the local lords with his feats of magic. He could then set out to unite the kingdoms under his rule, using a combination of wizardry and political savvy that he learned from his time at Hogwarts and in the wizarding world.
The White Walkers could be major antagonists. Naruto would have to use his skills to combat their undead hordes. Their magic is very different from the chakra - based abilities Naruto has, so it would be a great challenge.
Definitely Harry Potter himself. His unique magical abilities and his sense of justice would make him a very interesting character in the world of Westeros. Also, characters like Dumbledore could be interesting if they were somehow brought into the story. In Westeros, characters like Tyrion Lannister could have some really interesting interactions with Harry. Tyrion's wit and political acumen combined with Harry's magic would make for some great scenes.
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.
Naruto would probably find the social hierarchy in Westeros quite strange at first. But his friendly and adaptable nature would help. He might start at the bottom, but his skills would quickly earn him respect. He could become a sort of outsider hero, not fully bound by the strict rules of the hierarchy.