A powerful mage could be a great main character. They could use their magic to either combat the Four Kings or try to understand the source of their corruption. Magic in the Dark Souls world is often mysterious and having a mage as the main character could allow for exploration of different spells and magical concepts in relation to the Four Kings.
One idea could be exploring the backstories of the Four Kings before they became corrupted. Maybe they were once noble knights who were slowly influenced by the power they sought. Another could be a story where a new character tries to redeem the Four Kings instead of just defeating them. For example, this character could find a way to break the curse that binds them to their evil deeds.
After the Four Kings, you might want to make your way to Anor Londo. This area holds more challenges and secrets related to the main story. But you can also choose to backtrack and complete any side quests or collectibles you might have missed earlier.
In Dark Souls The Lost King story, the main character is the Lost King. His actions, his rise and fall, are what the story is mainly about. All the events in the story are somehow related to his decisions and his fate.
A brooding vampire could be the main character. He has lived for centuries and has seen the worst of humanity. However, he decides to use his superhuman strength and speed to protect a small town from other supernatural threats that lurk in the night.
Dark Souls 1's story has you exploring Lordran. You encounter various bosses like Gwyn, the Lord of Cinder. His actions in the past are related to the state of the world now. In Dark Souls 2, there are different areas like Heide's Tower of Flame. The story unfolds as you interact with the strange characters and face the challenges they present, all while trying to deal with the overarching curse that plagues the land.
Obviously Naruto himself would be a main character, in his dark form. There might also be other characters like Sasuke. Sasuke might be in a position of either trying to stop Naruto or joining him in his dark deeds. And Sakura could also be there, perhaps as a voice of reason trying to bring Naruto back from the darkness.
There's also the Fire Keeper. She plays a crucial part as she is associated with the bonfires and provides guidance to the Ashen One. And then there's the Soul of Cinder, which is a significant entity in the novel's plot. It represents the combined wills of all the Lords of Cinder who have failed in their duties to link the First Flame in the past.
A young orphan could be the main character. They are lured into the dark woods circus by the promise of a family and a place to belong. But soon they realize the dark secrets of the circus.
It could be a wealthy businessperson who has become blinded by greed in the cut - throat corporate world. They are the main character and the 'light in the dark' could be a poor but wise employee who shows them the error of their ways.
Sasuke could also be a main character in such a fanfiction. Given his brooding nature and his search for power, the addition of dark angel powers could take his character in an even more complex direction. He might be more tempted by the dark side that comes with the dark angel powers.