The plot could involve Naruto being an outcast in the ninja world after some false accusations. The dark guild offers him a place where he feels accepted. However, the guild's activities are against the values he once held. So he has to decide whether to stay and try to change the guild from within or break away and face the consequences of having associated with them in the first place. It could also lead to some interesting battles where Naruto uses his unique skills against his former comrades who now see him as a threat.
It could be the leader of the dark guild. This person would likely have a very different ideology from Naruto's. Maybe the leader believes in using extreme and unethical means to gain power, like sacrificing innocent lives for dark magic. Naruto would constantly clash with this leader as he tries to uphold his own values.
He might lose his friends at Fairy Tail. Since Fairy Tail is all about friendship and light, joining a dark guild would put him at odds with his former comrades.
She might lose her friends. In the original story, her friends in Fairy Tail are a big part of her life. If she joins a dark guild, they would likely see her as a traitor. For example, Natsu might be very disappointed and try to bring her back to the right side.
Her Celestial Spirit powers could be misused. Instead of using her spirits for good, like in Fairy Tail, she might be made to use them to intimidate or harm others. The spirits might also be reluctant to follow her commands if they sense she is on the wrong path.
Naruto would probably fit into the Fairy Tail guild quite well. He has a big heart and is always eager to make friends, which is a common trait in Fairy Tail. His loyalty would also be a great asset to the guild. He could be like the new, energetic member who is always ready for an adventure.
In some naruto dark guild fanfiction, there might be a plot where the dark guild is secretly working with rogue ninjas to create chaos in the ninja world. They could be trying to disrupt the balance of power between the villages. For example, they might start false rumors about one village attacking another to incite war. This would force Naruto and his friends to not only deal with the physical threats but also the political unrest caused by the dark guild's actions.
One possible fanfiction could be about a character from Naruto being transported to the Fairy Tail universe and getting involved with a Dark Guild. Maybe Naruto's skills in combat and his never - give - up attitude would help him either infiltrate the Dark Guild to gather information for Fairy Tail or try to reform some of the members.
In fanfiction scenarios where Naruto turns to the dark side, it could start with a personal tragedy. For example, if Hinata was killed by someone from the Leaf Village under false accusations. This could shatter Naruto's belief in the village and its values. He would then be vulnerable to the influence of the dark side. Once on the dark side, he could use his charisma to recruit others who have also felt wronged by the ninja world. He might form his own group, with different rules and goals from the traditional ninja villages. His journey to the dark side would not only change him but also send shockwaves through the entire ninja world, as he was once seen as a symbol of hope and a force for good.