In this fanfiction, the characters would have to adapt to a more cutthroat environment. Natsu's friends from Fairy Tail who might join him in the dark guild would struggle with their new identities. They would have to learn to use magic in more aggressive and unorthodox ways. Their moral compasses would be tested as they engage in activities that are considered 'dark' in the magic world. Some might even have a complete character transformation, going from good - natured mages to feared members of the dark guild.
Well, one possible plot could be that Natsu, after some sort of betrayal in Fairy Tail, decides to create his own dark guild. He might gather some disgruntled mages who also feel wronged by the mainstream guilds. Their goal could be to change the magic world's power structure in a more radical way, using darker forms of magic that are usually shunned. For example, they could explore ancient and forbidden magic spells to gain an edge over other guilds.
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.
Emotionally, Natsu could become more empathetic. If he meets characters in the dark guild who had a difficult past that led them to the dark side, he might start to see things from their perspective. This doesn't mean he would support their actions, but he would understand them better. For example, if there's a character in the dark guild who was orphaned and turned to dark magic for survival, Natsu might feel sympathy for them and try to offer an alternative path rather than just attacking them. This would show a more mature and compassionate side of his character that wasn't as prominent before.
Naruto himself would develop into a more authoritative figure. He would have to learn how to rule an empire, which is different from being a ninja. In the harem, his relationships with the women would make him more empathetic and understanding. For example, if he has a relationship with Karui, he might learn more about the Cloud Village's culture through her. His ninja skills might also evolve to be more about protecting the empire as a whole rather than individual battles.
Erza would be more composed. She would understand Natsu's reasons to a certain extent. However, she would also see the importance of the guild unity. So she would try to balance between respecting Natsu's decision and finding a way to make him come back if possible. She might send out some members to keep an eye on Natsu from a distance.
Natsu would likely develop a more serious and responsible side. He's always been a bit of a hot - headed adventurer, but with Kagura pregnant, he'd need to think about protecting his family. For example, he might take on less dangerous quests and focus more on providing for Kagura. Kagura, on the other hand, could become more nurturing. She's a strong fighter, but now she has to think about the life growing inside her.
Natsu's character could develop by learning more about the history of magic and his own dragon - slayer powers. In a time - travel scenario, he might discover new ways to use his magic or even find out some secrets about his past that were long lost. This new knowledge would make him more powerful and also more mature as he realizes the importance of certain things in the grand scheme of things.
In this fanfiction, Natsu could start to show a more mature and caring side towards Cana. He may become more attuned to her needs, like making sure she has proper food and rest. Cana, in turn, might grow in confidence as she prepares to be a mother. She could also become more assertive in protecting her unborn child from any potential threats in their magical world.
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 guild would likely be overjoyed. They would probably throw a big celebration with lots of food and drinks. Some members, like Erza, might be very protective and offer to help with babysitting or giving parenting advice. Gray would probably make some sarcastic but well - meaning comments about Natsu becoming a dad.