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.
The characters in Fairy Tail would have a wide range of reactions. Makarov, as the guild master, would be very cautious. He would need to ensure that Acnologia's presence doesn't put the guild in more danger. Happy would be constantly on guard around Acnologia, ready to fly away at the first sign of trouble. The Thunder Legion would likely be in a state of high alert. However, some of the more adventurous members like Cana might see it as an opportunity for great adventures and powerful new magic. It would be a complex situation with different characters having different stances based on their personalities and past experiences.
Some members might be initially cautious. Given her past as a powerful and somewhat feared being, they would be worried about her true intentions. For example, Gray might be on guard until he sees her actions are genuine.
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, 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.
The power balance would shift significantly. Acnologia is extremely powerful, so the guild would become a much more formidable force. There would be a lot of unease among the members at first, especially those who have heard of his past deeds. But his presence could also inspire some members to train harder to reach his level. The guild's reputation would also change, as having Acnologia in Fairy Tail would attract both fear and respect from other guilds.
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.
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.