Naruto himself would be an obvious choice as a main character. His ability to make friends and his never - give - up attitude would be great in a new and different empire setting. He could be the one to bridge the gap between the shinobi world and the new empire.
Sasuke could also be a main character. His complex personality and powerful skills would add a lot of depth to the story. In a crossover, he might be more intrigued by the power structures or dark secrets of the empire and get involved in some dangerous investigations.
Obviously Naruto would be one of the main characters. And from 'The 100', Clarke would likely be a main character as well since she is a central figure in her own show.
Perhaps an ancient evil that has been sealed away in the empire for centuries but is released due to the chaos of the crossover. This entity could be more powerful than any foe the Naruto characters have faced before, forcing them to find new ways to fight.
A group of ninja who oppose the new order of the Naruto Empire could be the main antagonists. They might believe that the empire has become too controlling or has strayed from the original values of the ninja world. They could use guerrilla tactics and create internal unrest within the empire to try and bring it down.
A politician who drives the empire - building policies could be a main character.
Definitely Naruto himself. He's the main character of the Naruto series, so he has to be there. In a crossover with Attack on Titan, he could meet Eren. Naruto's determination and Eren's drive to protect his people could make for an interesting dynamic between them.
There might be some original characters created specifically for this fanfic. They could be characters that act as guides or rivals within the game - inspired aspects of the story. For example, a mysterious figure who gives Naruto quests or a fellow ninja who is competing with him for high scores in the game - like system.
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.
Definitely Naruto would be a main character. He is the protagonist of the original series and would be the one to drive the story forward in the crossover. Then, perhaps a new character could be introduced who has a special connection to the bone - related aspect. This character could be from another world or a long - lost clan in the Naruto universe. Also, Gaara could be a main character. His sand powers could interact in interesting ways with the bone - related elements, like his sand protecting him from bone attacks or being able to incorporate bones into his sand defenses.
Obviously, Naruto would be one of the main characters. He's the protagonist of the Naruto series and would be the main link to the ninja world in this crossover. Another could be a young Kitsune, maybe one that's just coming of age in the Kitsune world. Their paths cross, and they have to learn to work together.
Obviously Naruto himself would be a main character. He could be joined by Sasuke. In a crossover with 'One Piece', Luffy could also be a main character in the fanfiction, as he is the captain in 'One Piece' and would bring his own unique charm and abilities.