If Natsu were trained by Acnologia, he might face a moral dilemma. Acnologia is a feared and often seen as an evil force. Natsu has always been on the side of good. So, during the training, he would have to decide whether to follow some of Acnologia's more ruthless ways of using power or stay true to his own values. This internal struggle could add a lot of depth to his character. Also, his relationships with his friends in Fairy Tail might be strained as they might be worried about the influence Acnologia has on him.
In many Fairy Tail fanfictions, the battle between Natsu and Acnologia is often depicted as extremely epic. Natsu's fiery determination and his unique dragon - slayer magic make for an interesting contrast against Acnologia's overwhelming power. Some fanfictions might have Natsu discover new forms or powers in order to stand a chance against the fearsome Acnologia.
Some writers characterize Acnologia as a misunderstood loner. They show that his actions of destruction are a result of his past traumas. Others might make him more of a power - hungry villain who just wants to rule over all. There are also those who give him a sense of humor, which is a very different take from his usual brooding and menacing self in the original series.
A common theme in such fanfictions is Natsu's unwavering courage when facing Acnologia. He might not be as strong as Acnologia at first, but his will to protect his friends and the Fairy Tail guild drives him forward. His dragon - slayer magic could potentially have some hidden aspects that are only revealed during this encounter.
Well, Naruto would probably gain some really unique magical abilities. Fairy Tail mages are known for their diverse magic, and Naruto could combine his chakra skills with something like Fire Magic or Celestial Magic. He might become even more powerful in combat, with new ways to attack and defend.
The guild as a whole would likely mobilize. Erza would start formulating a strategic plan. She'd consider the strengths and weaknesses of Acnologia and figure out the best way to approach the situation. Gray would also be ready to fight, using his ice - make magic to support the rescue mission. They would all be united in their determination to get Lucy back safely.
One possible plot could be that Acnologia, with his immense power, initially causes chaos within Fairy Tail. The members are both in awe and afraid of him. But as time passes, he starts to understand the concept of friendship and loyalty in Fairy Tail. He could then use his power to protect the guild from other major threats, like the Alvarez Empire. This would completely change the power dynamics in the Fairy Tail world.
Well, adapting to Acnologia's body could be a complex process. They would have to learn how to control the raw power within it. This power might be overwhelming at first, and they could accidentally cause destruction. To adapt, they might start by testing their limits in isolated areas. Emotionally, they would also have to come to terms with the fact that they are now in the body of a feared being. They might try to use their memories of Fairy Tail to bring out the good in this new form, like using Acnologia's strength to protect the weak as they did in their previous life.
There could be a lot of character development. Maybe she has a troubled past that she shares with the members of Fairy Tail, which helps her grow closer to them and become a more empathetic character. She might also learn the value of friendship within the guild, which could change her whole perspective.
Well, in some fanfictions, the plot revolves around Acnologia's redemption. He might start to see the value in the world he once wanted to destroy. This could be due to a series of events that make him realize there's more to life than just being a force of destruction. Also, a common plot is his relationship with other dragon slayers. Some fanfics explore how he interacts with them, whether it's a rivalry or an unexpected alliance.