She may develop new powers or a different way of using her existing powers. The dark guild might have some dark magic or techniques that they force her to learn. This could potentially make her powers stronger but in a more dangerous and unethical way.
There could be a struggle within her powers. Since her powers are based on a connection with her spirits, which are generally good - natured, her alignment with a dark guild could create an internal conflict. This might lead to her powers being unstable or not working as effectively as before.
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.
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.
It's possible that her powers could actually get stronger in some ways. Since pregnancy is a big physical change, her body might adapt by enhancing her powers. But this could also be dangerous as she may not be used to the increased strength.
His character could become more conflicted. He has to balance his old values from Fairy Tail with the new, darker ones of the guild he joined.
She would become more cynical. Instead of seeing the good in people, she would start suspecting everyone's motives. For example, she would no longer trust her former friends easily.
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.
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.
Her relationship with Natsu would likely change. Natsu might be initially distrustful of Lucy if he senses her connection to End, since End is often seen as an enemy. But then as he realizes that Lucy is still the same person deep down, their bond could become even stronger as he helps her control this new aspect of herself.
Ichigo's powers might be enhanced or altered. Since he has a unique combination of Hollow, Shinigami, and Quincy powers, time travel could disrupt the balance. For example, his Hollow side could become more dominant if they travel to a time when Hollows were more powerful. His Zanpakuto might also change in appearance or abilities.
Percy's powers could potentially be weakened because his energy is being diverted to support the pregnancy. He might not be able to perform his usual feats as easily. Artemis, on the other hand, might find that her powers become more erratic. Her silver arrows could lose some of their accuracy, or her ability to communicate with animals could be distorted. This could lead to some interesting situations where they have to rely on each other more than their powers.
She might start by having Lucy grow disillusioned with the normal guilds. Maybe she's been mistreated or feels restricted. So, she decides to gather like - minded mages who also have issues with the existing guild system. They could start small, doing jobs that are a bit on the edge of legality. As they gain power and members, they face opposition from the regular guilds. This could lead to epic battles and a lot of character development for Lucy as she struggles between her old ideals and the new, darker path she's chosen.