One interesting concept could be how the dark chakra affects a character's personality. For example, it might make them more aggressive or unpredictable. Another could be its origin, like if it's from an ancient curse or some dark experiment.
One possible storyline could be that Naruto struggles to control this dark chakra. He might accidentally harm his friends and then have to go on a journey to find a way to suppress or purify it. For example, he could seek out ancient jutsu or visit reclusive sages.
In Naruto dark chakra fanfiction, Sasuke is frequently a central figure. Given his complex character arc and his inclination towards power - seeking, it's easy to envision him being affected by or exploring dark chakra. Naruto too can be a main character. His strong will and connection to positive chakra could be put to the test when faced with the allure of dark chakra. Another character often involved is Kabuto. He has a history of being involved in dark and underhanded schemes, so it's not a stretch to see him in stories centered around dark chakra. There are also some original characters created by fanfiction authors that are specifically designed to interact with the concept of dark chakra in unique ways.
It could be a rogue ninja who is also after the power of the chakra chains. This ninja might see Naruto as a threat and try to eliminate him to get the chains for himself.
One idea could be Naruto using the chakra chains in a unique way to fight a powerful new enemy. For example, he might combine them with his shadow clones to create a multi - pronged attack. Another could be the origin of these dark chakra chains, perhaps they are linked to an ancient and evil power within Naruto that he has to learn to control.
In 'naruto dark chakra fanfic', perhaps an interesting aspect is the unique jutsu Naruto could develop with the dark chakra. It could be a new form of Rasengan that has a darker, more menacing look and stronger power.
It could make him more brooding. Instead of his usual cheerful self, the dark chakra might cause him to be more withdrawn as he grapples with this new power within him.
One consequence could be that he becomes extremely powerful. His physical abilities like strength, speed, and endurance are enhanced. He can also use more powerful jutsus with the extra chakra boost from the Kyuubi.
One interesting way is when Naruto absorbs chakra through his shadow clones. In some fanfictions, the clones can be used not just to gather information or attack, but also to absorb the chakra of the opponents they interact with. Then, they transfer this absorbed chakra back to Naruto, making him more powerful.
Perhaps there is an ancient artifact or a powerful curse that causes Naruto to absorb all chakra. In the fanfiction world, it's common to introduce such unique elements. For example, he might find a magical scroll that, when opened, starts sucking in all the chakra in the vicinity and then channels it into Naruto. Or it could be a curse placed on him by an enemy, but instead of harming him, it gives him this strange power to absorb all chakra.