Sakura could also be a main character in such a fanfiction. As a highly skilled kunoichi, she could accompany Naruto to the Kage Summit. She might use her medical ninja skills to help in case of any emergencies during the summit and also play a part in diplomatic discussions, especially when it comes to matters of ninja welfare and healthcare in different villages.
Naruto could still be the main character. As the Hokage, he would be at the center of the Kage Summit. His ideals of peace and unity would play a major role in the interactions with the other Kages. He might try to convince the other Kages to support a new initiative for a more united ninja world.
It could be a new character, a young ninja who is sent to the Kage Summit as an observer or assistant. This character could witness all the political drama and power plays at the summit. They might end up having a crucial role, like uncovering a conspiracy that could endanger all the villages.
Perhaps in the fanfiction, there are internal power struggles within the villages that are revealed at the Kage Summit. A young ninja from one of the villages might try to influence the decisions of their Kage during the summit. There could also be a cultural exchange aspect, where the Kages learn about each other's villages' traditions and find ways to incorporate beneficial elements into their own villages.
A new, young ninja from a minor village could be the main character. They could be trying to make a name for themselves and their village at the Kage Summit. They might get involved in some political intrigue or even a fight between the Kages and prove their worth.
It could be Naruto himself. Since he has a connection to all the villages in some way, he could play a crucial role in the fanfic, perhaps as a mediator or someone who uncovers a hidden conspiracy during the summit.
It could be a rogue ninja who has a vendetta against all the villages represented at the summit. He might have been wronged in the past by the combined actions of the five great villages and now seeks revenge. He could use the chaos of the summit to launch attacks and try to undermine the stability of the shinobi world.
The Kages themselves are often main characters. For example, Tsunade from Konoha, Gaara from Suna, and the other village leaders. They play important roles in the political and power - related discussions during the summit.
Sasuke could be one to stop Naruto. They have a complex relationship and Sasuke might understand Naruto's motives and be able to talk him down.
Other villages' leaders or powerful ninjas who don't believe Naruto is ready to be a Kage. They might try to test him or even plot against him during diplomatic meetings before the summit, seeing his new position as a potential weakness in the balance of power.
Obviously Naruto would be one of the main characters as the title implies he's the one attacking. And the Kages, like Gaara, Tsunade, and the other village leaders would also be main characters since they are at the summit that Naruto is attacking.
Gaara could be one of his potential allies. Since they have a history together and Gaara has changed a lot, he might support Naruto. Also, the Raikage might become an ally if Naruto can prove that his intentions are good for the entire ninja world.
In a 'Dark Naruto Kage Summit' fanfiction, it's possible that Dark Naruto is actually a misunderstood figure. He arrives at the summit not to cause destruction but to seek help. His dark transformation might be the result of an experiment gone wrong that he had no control over. The Kages, initially seeing him as a threat, might come to realize his true situation after he reveals his story. This could lead to a different kind of relationship between Naruto (even in his dark state) and the Kages, where they work together to find a solution to his problem.